Method and apparatus for active control of golf club impact

ABSTRACT

A method and apparatus for actively controlling the impact between a club head and a golf ball. A golf club head has a face with an actuator material or device mechanically coupled to influence face motion. The face actuation controls impact parameters, impact properties, or resulting ball parameters such as speed, direction and spin rates resulting from the impact event between the face of the club and the golf ball. Further, the apparatus has a control device for determining the actuation of the face. Several embodiments are presented for controlling parameters such as ball speed and direction. The invention can use energy derived from the ball impact, converted into electrical energy, and then reapplied in a controlled fashion to influence an aspect of the face, such as position, velocity, deformation, stiffness, vibration, motion, temperature, or other physical parameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of currently-pending U.S.Non-provisional patent application Ser. No. 10/915,804, filed Aug. 9,2004, which claims priority from U.S. Provisional Patent ApplicationSer. No. 60/494,739 filed Aug. 14, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the field of advanced sportingequipment design and in particular to the design and operation of a golfclub head system for control of the impact between a club head and agolf ball.

2. Background Art

The present invention pertains to achieving an increase in the accuracyand distance of a golf club (e.g., a driver) through the application ofcontrols techniques and actuation technology to the design of the club.There have been many improvements over the years which have hadmeasurable impact on the accuracy and distance which a golfer canachieve. These have typically focused on the design of passive systems;those which do not have the ability to change any of their physicalparameters under active control during the swing and in particularduring the impact event with the golf ball. Typical passive performanceimprovements such as head shape and volume, weight distribution andresulting components of the inertia tensor, face thickness and thicknessprofile, face curvatures and CG locations, all pertain to the selectionof optimum constant physical and material parameters for the golf club.The present invention pertains to the development of an active systemwhere critical parameters of the golf club and head (for example surfaceposition/shape/curvature or effective coefficient of friction, or facestiffness) can be selectively controlled in response to the actual stateof the physical head-ball system. Such states can be head velocity,impact force, intensity, impact duration and timing, absolute locationof the head or relative location of the ball on the face, orientation ofthe head relative to the ball and swing path or parameters, physicaldeformation of the face, or any of numerous physically or electricallymeasurable conditions.

The present invention relies on the field of controls technologies andin particular structural or elastic system actuation technologies andcontrol algorithms for such systems. See for example: Fuller, C. R. etal., Active Control of Vibration Academic Press, San Diego, Calif. 1996.A particular embodiment of one controlled system relies on frictioncontrol using ultrasonic vibration (Katoh). An alternate embodiment ofone controlled system relies on changing the effective stiffness of theface to control impact with the ball. The present invention also relieson the concept of piezoelectric energy harvesting and/or simultaneousenergy harvesting from and actuation of mechanical systems.Piezoelectric energy harvesting is described in the following U.S. Pat.Nos. 4,504,761; 4,442,372; 5,512,795; 4,595,856, 4,387,318; 4,091,302;3,819,963; 4,467,236; 5,552,657; and 5,703.474.

The impact between the ball and the head can be interpreted in terms ofthe idealized impact between two elastic bodies each having freedom totranslate and rotate in space i.e. full 6 degrees of freedom (DOF)bodies, each having the ability to deform at impact, and each havingfully populated mass and inertia tensors. The typical initial conditionfor this event is a stationary ball and high velocity head impacting theball at a perhaps eccentric point substantially on or substantially offthe face of the club head. The impact results in high forces both normaland tangential to the contact surfaces between the head and the ball.These forces integrate over time to determine the speed and direction,forming velocity vector and spin vectors of the ball after it leaves theface, hereafter called the impact resultants. These interface forces aredetermined by many properties including elasticity of the two bodies,material properties and dissipation, surface friction coefficients, bodymasses and inertia tensors.

Some of these properties and conditions of the face can be activelycontrolled during the impact resulting in some measure of control overthe impact resultants. For example, in a specific embodiment, thesurface can be ultrasonically vibrated under some predeterminedcondition so as to create an effectively lower friction coefficientbetween the ball and the face resulting in decreased spin rates andlonger flight of the ball when a trigger condition is present. One suchtrigger condition might be high head ball impact forces (and large facedeformation), indicating a high velocity impact where too much spincould create excess aerodynamic lift producing a decreased flightdistance.

In another embodiment, the position and/or orientation of the face canbe actively controlled relative to the ball and the body of the clubunder some predetermined condition so as to create a better presentationof the face to the ball for more accurate ball flight or to reduce sidespin by counteracting club head rotation during eccentric impact events.One such triggering condition might be highly eccentric impact events(off center hits) that can be detected by deformation sensors on theface or angular acceleration sensors in the body. Such sensor signalscould be processed to determine the necessary motion of the face tocompensate and correct the resulting ball flight.

In another embodiment, the effective stiffness of the face during impactcan be controlled so as to produce a more desirable impact event. Forexample, the system can be designed to make the face stiffer during ahard impact and make the face softer during a less intense impact so asto tailor the face behavior under the impact loads to the particularevent. This can be accomplished by, for example, shorting or opening theleads of a piezoelectric transducer which has been surface bonded orotherwise mechanically coupled to a face. The piezoelectric is softer(low modulus) when it is electrically shorted and stiffer (higheffective modulus) when it is open circuited. A sensor attached to theface can measure a quantity proportional to impact intensity (e.g., facedeflection, face strain, head deceleration etc). In the “hard” hit case,the normally shorted piezoelectric can be open circuited to make theface stiffer, while softer hits result in the circuit leaving thepiezoelectric in the short circuited condition and therefore less stiff.

The trigger can be provided by an external sensor or by the actualpiezoelectric transducers bonded to the face itself by triggering off ofthe current or voltage level achieved on the transducer prior to thetriggering event. As an example, circuitry for using the piezoelectricelement as a charge sensor can be attached to the transducer leads. Whenthe charge reaches a critical level a circuit can be triggered whichdisconnects the leads from the circuitry effectively enforcing the opencircuit condition.

A critical element of the ability to control the ball-head impact is theability to actuate the system in a beneficial manner. Since the head andball are a mechanical system, this entails the application of some forceor thermal energy to the system so as to create a change in somemechanical physical attribute. The present invention pertainsprincipally to mechanical actuation techniques.

U.S. Pat. No. 6,102,426 to Lazarus, et. al, discloses the use of apiezoceramic sheet on a ski to affect its dynamic performance such aslimiting unwanted vibration at higher speeds or on irregular surfaces:The disclosure mentions the application to golf clubs to dampenvibrations or alter shaft stiffness or “to affect its head”.

U.S. Pat. Nos. 6,196,935, 6,086,490 and 6,485,380 to Spangler et. al,disclose the use of piezoceramic sheets on golf clubs to alter stiffnessand to effect a dampening of vibration. FIG. 9G illustrates theplacement of piezo elements on a golf club head to capture strain energyto be dissipated in a circuit for a dampening effect.

U.S. Pat. No. 6,048,276 to Vandergrift relates to the use ofpiezoelectric devices to stiffen the shaft of a golf club aftercapturing energy from the swinging and flexing of the shaft.

The issue of reducing friction using ultrasonic vibration is discussedby Katoh in an article entitled “Active Control of Friction UsingUltrasonic Vibration” Japanese Journal of Tribology Vol. 38 No. 8 (1993)pp 1019-1025. See also K. Adachi et al “The Micromechanism of FrictionDrive with Ultrasonic Wave”, Wear 194 (1996) pp 137-142.

SUMMARY OF THE INVENTION

The present invention pertains to a system for the control of the impactevent between the ball and the club face using actuation and control ofthe face position or properties to influence the progression of theimpact event between the ball and the face. In particular, it pertainsto the reuse of energy generated and converted to electrical energy fromthe mechanical energy of the impact event. Such reuse beneficiallycontrols the impact event. In a particular embodiment, the energyconverted from impact by a piezoelectric element is converted intoultrasonic face deformations/oscillations which have the ability toeffectively lower the friction coefficients between the ball and theface. In an alternate embodiment, the stiffness of the piezo-coupledface under impact is controlled to a certain behavior upon theoccurrence of predetermined impact parameters. For example, the face ismade stiff under hard hits and soft under lower intensity hits. Allthese cases pertain to putter, drivers and irons equally and club-headwill be taken to mean all of these without prejudice.

The face actuator can be any of a number of actuators capable ofconverting electrical energy to mechanical energy. These includeelectromagnetic types such as a solenoid, as well as a family ofactuation technologies using electric and magnetic induced fields toeffect material size changes; electrostrictive, piezoelectric,magnetostrictive, ferromagnetic shape memory alloys, shape memorymagnetic and shape memory ceramic materials, or composites of any of theabove. Included in the possible actuation schemes are thermal actuatorsusing resistive heating or shape memory alloys which use applied thermalenergy to induce a phase change within the material to induce aresulting size change or stress. All can be used to convert electricalenergy into face deformation or face positioning in a controlledfashion.

In such a system using a pure actuator there must be an electric energysource, battery or other electrical generator converting motion orimpact energy into the electrical energy which is used by the faceactuator. The system can include a power source, electronics, and anactuator mechanically coupled to the head.

In a further definition there is alternately a class of system in whicha transducer is coupled to the face. A transducer is capable ofgeneration of electrical energy from mechanical energy as well as viceversa. Examples of transducer materials include electromagnetic coilsystem, piezoelectric and electrostrictive materials operating under abiased electric field, and magnetic field biased magnetostrictivematerials and ferromagnetic shape memory alloy materials, and orcomposites of the above with themselves or other constituents. Thesewill hereafter be called piezoelectric materials generally and the useof the word piezoelectric shall in no way be taken as limiting. Insystems employing such transducers, the transducer element can becoupled to the face such that deformation or motion of the clubgenerates electrical energy which can be used via the converse actuationfunction to control aspects of the head-ball impact.

Piezoelectric actuators are the most common of the class of transducermaterials. In general, they change size in response to applied electricfield and conversely they generate charge in response to applied loadsand stress. They can be used both as electrically driven actuators andelectrical generators.

Control of the impact involves putting forces on the head and/or face soas to beneficially change a property of the system which influences theimpact event. For example, if the force applied is proportional to theface-acceleration, then the control acts to apparently increase the massor inertia of the system. It does this by putting the same force on thehead that a mass at that location would put under that particular facemotion. The applied force can be applied to effectively create forceswhich mimic elastic and dissipative as well as inertial forces of thesystem. For example, if the force put in the center of the face were tobe proportional to the velocity and opposing the velocity at the centerof the face, then it would effectively act as a dashpot at the center ofthe face and create a viscous damper at the center of the face.Similarly, if one could apply a force which was essentially proportionaland opposed to the deflection of the center of the face, then it wouldlook like a spring applied at the center of the face-effectivelystiffening it. Likewise if the force was proportional and in thedirection of the deflection then it would look like a negative springapplied at the center of the face—effectively softening the face. Theactively controlled system (if one can control the force), can mimicmany different dynamic effects in the system. The challenge is todevelop a device and system which can put those types of forces on thesystem even if some other constraints prohibit that.

The idea of applying some forces that mimic other types of forces thatwould result from inertias or masses, is one manifestation of the forcesthat can be applied. In such control systems there can be an arbitraryphase relationship between the applied force and input and thatrelationship can be frequency dependent. Essentially the controlfunction can be a linear or nonlinear dynamic system between some sensorand the output force applied by the actuator. In a classic controlledsystem, there is a control system which takes sensor outputs and putsforces on the body to achieve some desired effect. That's the generalarea of dynamic systems control and more specifically, structure controlfor elastic systems and is well defined in the art.

Ultrasonic, or high frequency, oscillations of contacting surfaces canresult in lower effective coefficients of friction between the twosurfaces. The oscillations must be of sufficient amplitude and frequencysuch that the surfaces lose contact briefly during at least one portionof the oscillation. This breaking of contact lowers the effectivecoefficient of friction.

An actuator coupled to the club face can be configured to excite highfrequency oscillation of the face when driven with high frequencyelectrical input. If the excitation occurs at a frequency at or near aresonant frequency of the club/face body, then the amplitude can bemaximized.

In scenarios such as a golf ball impact where the normal forces are highduring impact, the key requirement is that the acceleration of the faceaway from the ball during the oscillatory motion should be high enoughthat the ball cannot “catch up” and surface contact is broken. Theacceleration is proportional to the amplitude of the oscillatory motionmultiplied by the square of the excitation frequency. This can beconsidered a figure of merit of the design of the actuation system.Since the amplitude of oscillation for an actuated system tends to rolloff due to system inertial effects, there is a tradeoff between drivingat higher frequency and achieving the highest possible oscillatoryamplitude. The figure of merit helps balance these to maximize thefriction control effect. For example, in the preferred embodiment of thepresent invention, it was found advantageous to excite a face surfacemode at 120,000 Hz which is coupled to the actuation driver describedhereinafter.

In systems where an external source of power is not available, a portionof the energy of impact (converted from mechanical to electrical by atransducer coupled to the face) can be stored and returned to the facein the form of ultrasonic excitation of a high order face mode, highfrequency oscillations of the face which are well coupled to thetransducer. The energy can be stored in the transducer material itself,for example in the charge stored in the capacitance of a piezoelectricmaterial or it can be stored primarily in auxiliary circuit elementssuch as storage capacitors or inductors or tank circuits, etc, which areelectrically coupled to the transducer. After a triggering effectreleases the energy, an electrical drive circuit can be configured sothat when connected to the transducer, it induces a high amplitude faceoscillation which effectively reduces the impact friction coefficientbetween the ball and the face at a critical point in time during theimpact event such critical point in time being selected by a controlalgorithm. The face oscillation and controlled friction result in acontrol of ball spin which can be selectively triggered under certainimpact conditions (such as high impact force levels).

The exiting ball speed can also be controlled by applying forces to theface proportional to face deflection. With appropriate sign these forcescan effectively soften the face by increasing the duration of the impactthereby lessening the impact loading and resulting ball deflection. Thelower ball deflection results in reduced dissipation by inelasticdeformation of the ball and increased recoverable energy from the impactevent, thus achieving higher coefficients of restitution (COR) andhigher ball velocities. Conversely, impact energy converted intoelectrical energy can be dissipated to decrease the effective COR inselected impact scenarios.

By selectively applying forces electrically to mimic the effects oftailored compliance, portions of the face can be selectively made todeform greater than others during the impact event thus controlling theexit direction of the ball. The exit direction is controlled because thefinal ball velocity (speed and direction) is determined by the forcesgenerated by the elastic impact. Uneven deformation of the face (due tounbalanced compliance) changes the direction of the normal reaction ofthe ball and therefore the final direction the ball will travel. Inaddition to this direct control of ball direction, indirect control ofball direction can be achieved by reducing spin including sidespin andthereby reducing cross range travel. Similar control features can beachieved by actively positioning an actuated clubface during impact inresponse to some measured impact variable such as location of the impactor angular acceleration of the head (caused by eccentric impact).

Forces can also be applied to the head to mimic the effects of a highermoment of inertia. In other words, the forces would be similar to thosethat an additional mass at a given location would exert on the headduring impact. Such forces can be triggered in miss hit scenariosresulting in straighter shots. For instance, one way of doing that wouldbe to create a force on the head through action with a reaction mass.The actuator reacts between the head and the reaction mass. It reacts insuch a way that it minimizes head rotation under impact. It acts toeffectively increase the moment of inertia of the body and thereforekeeps the face straighter and therefore the ball flight straighterduring the impact event. Because the impact event is of a finiteduration, one can put that kind of force on the body within that finiteduration. A central post and an annular bimorph ring would be segmentedso that one can actually detect and sense which way the head is movingrelative to the reaction mass. Whether it is up, down, left or right,basically which way the face is rotating could be used as a sensor inputto a compensator/controller to allow the applied force to compensate forthat resulting face motion. Multiple piezo elements or configurationswith multiple electrodes on a single piezoelectric element would allowdetection of a broader range of impacts. One can actually determinewhere the ball is impacting on the face and use the control circuitry tocompensate accordingly, for instance by slightly rotating the face tocompensate for head rotation during eccentric impacts. In the preferredembodiment there is one voltage coming out of one piezo making itdifficult to determine the impact location from the variety of possibleimpact locations. But that is not necessarily a limitation of thepresent invention. It is possible to include a uniform piezo bonded tothe face where the electrodes are segmented to allow detection of impactlocation. In that scenario, essentially there would be multiplepiezoelectric elements that are bonded to the face. There would bemultiple electrodes for example in a square array. For example theremight be actually nine electrode patterns in a 3×3 square array on theback of the face. Those voltages would be applied to a control circuitthat would determine where the ball has impacted and the resultingappropriate response to that impact. Switching on the voltage on some ofthe electrodes on the transducers as opposed to others in response,could tailor the response depending upon impact location.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments, features and advantages of the presentinvention will be understood more completely hereinafter as a result ofa detailed description thereof in which reference will be made to thefollowing drawings:

FIGS. 1-5 illustrate various conceptual embodiments of the inventionwherein different forms of elastic coupling of a piezoelectric actuatorto a golf club head face are shown;

FIGS. 6-8 illustrate various conceptual embodiments of the inventionwherein different forms of inertial coupling of a piezoelectric actuatorto a golf club head face are shown;

FIG. 9 illustrates a conceptual embodiment of the invention whereinpiezoelectric transducers are disposed between the face and body of theclub positioning the face relative to the body;

FIGS. 10 a and 10 b are a block diagrams of a piezo actuator withcontrolled switch, inductor, and control circuit;

FIG. 11 is a schematic diagram of the circuit of FIG. 10 b showing thecontrol circuit in more detail;

FIG. 12 is a graphical presentation of an actuator output voltage signalunder ball impact showing un-triggered and triggered voltage timehistories;

FIG. 13 is a graphical presentation of the time histories of keyparameters in the ball to club impact showing A) impact normal force, B)impact tangential (friction) force, C) transducer voltage timehistories, D) transducer current time histories, and E) resulting ballspin time histories;

FIGS. 14-15 are section illustrations of a golf club head employing theconceptual piezo coupling embodiment of FIG. 2 to reduce the spin rateof a golf ball by converting ball impact energy into a head facevibration to reduce friction between the head and the golf ball;

FIGS. 16 a and 16 b together comprise an illustration of a golf clubhead employing the conceptual piezo coupling embodiment of FIG. 2detailing the removable sole plate with system electronics;

FIGS. 17-19 are detailed illustrations of the face assembly showingpiezoelectric transducer to face coupling hardware for conceptualpiezoelectric coupling embodiment of FIG. 2;

FIG. 20 is a graphical presentation of the friction model for theinteraction between the face and the ball;

FIG. 21 is a frequency response function showing the voltage response ofan open circuit piezoelectric transducer undergoing periodic loading onthe face of the club;

FIG. 22 is a frequency response function showing the face surfaceacceleration as a function of the amplitude of time varyingvoltage-excitation of the piezoelectric transducer; and

FIG. 23 is a circuit block diagram of a electrical system for achievingvariable stiffness which stiffens upon mechanical excitation of thepiezoelectric of sufficient intensity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following description assumes that there is an understanding of thefundamentals of piezoelectric materials, operations and modes such asdescribed in “Piezoelectric Ceramics” by Jaffe, Cook and Jaffe, AcademiaPress, 1971 and the references cited therein. The content of thatpublication is hereby incorporated herein in its entirely by reference.Another useful reference which describes the field of piezoelectricmechanics is “Piezoelectric Shells” by H. S. Tzou, Kluwer, AcademicPublishers, MASS., 1993 and is also hereby incorporated herein byreference.

Actuator Coupling to Face

There are several methods of coupling actuation elements and transducersto the club face, the interaction surface between the ball and the head.The transducer can be directly coupled to 1) the face relativedeformation (elastic), 2) absolute motion (inertial) using a variety oftechniques or 3) relative motion between the face and the head body.Eight are described here which alternately couple the actuator ortransducer to elastic deformation of the face or inertial motion of thehead. For the actuation function the goal is to enable maximal controlover face deflection at the desired frequency of actuation. For thetransducer, the goal is to maximally couple into either the absolutemotion (deceleration) of the head (or face) or into the deformationpattern induced in the head and face by the ball impact. The twotechniques tap into the pool of kinetic or elastic energy availableduring impact. This energy is then converted by the transducer intoelectrical energy which is usable for face and interface actuation. Adescription of eight alternative systems for coupling a transducerelement to the golf club face follows.

There are three classes of actuator face coupling. The first classpertains to elastic piezo face actuation wherein transducer size changesand deformations are directly mechanically coupled into relativedeformation along or between two structural points on the face. Thistype of elastic actuation is generally known in the art of structuralcontrol where piezoelectric elements (predominately) are mounted on orembedded within structures to effect beneficial structural deformation.The four embodiments of elastically coupled actuators are as follows:

-   -   Concept 1—Piezo wafer attached directly to the face to actuate        bending as shown in FIG. 1.    -   Concept 2—Piezo stack and/or tube mounted on the face with        housing as shown in FIGS. 2 a, 2 b and 3.    -   Concept 3—Piezo disposed between the face and a stiff backing as        shown in FIG. 4.    -   Concept 4—Piezo operated in shear mode and disposed between the        face and a stiff constraining layer as shown in FIG. 5 a 5 b.

The second class of actuator face coupling is actuator coupling to theface's absolute motion or those that rely on inertial forces generatedby face and head motion on impact with the ball. These typically entaila reaction mass and an actuator or transducer element acting between thereaction mass and the face. These types of face couplings are generallyrelated to proof mass or reaction mass actuators. The concepts in thiscategory are described as follows:

-   -   Concept 5—Direct piezo coupled between the face and an inertial        mass as shown in FIG. 6.    -   Concept 6—Motion amplified piezo between the face and an        inertial mass as shown in FIG. 7.    -   Concept 7—Bimorph type piezo with tip mass and mounted on the        face as shown in FIG. 8.

The third class of actuator-face coupling is actuator coupling betweenthe face and the body of the club. The actuator can be the sole or oneof a number of parallel load paths between the face and the body. Thisis similar to Concept 3 but the face is treated more like a rigid bodythat can be positioned rather than deformed as in Concept 3. Thetransducer positioned between the face and the body supports themajority of the loads between the face and the body and can thereforeparticipate to a large extent in the impact event. In addition,actuation induced positioning of the face relative to the body inessence uses the body itself as a large reaction mass to effect changesin the location or orientation of the face during impact.

-   -   Concept 8—piezoelectric transducer positioned between the face        and body of the club as shown in FIG. 9.

For transducer applications, to produce maximal available actuationpower and maximally available coupling (for instance actuating highamplitude high frequency face oscillations for spin control) it isdesirable to achieve good coupling to both 1) impact deformation patternas well as 2) a high frequency mode. For face positioning applications(rather than friction reduction applications) it is desirable to achievegood coupling to both 1) impact loading patterns as well as 2)impact-timescale motion between the face and the body.

In general for the elastically coupled concepts (1-4), facemotion/loading generates loading on the transducer material andcorresponding electrical energy generation. Conversely, electricalenergy put on the transducer controls face motion. It is desirable tohave high electro-mechanical coupling between face loading/motion andelectrical voltages and currents. This coupling can be measured in termsof the fraction of input mechanical energy from the impact that isconverted into stored electrical energy (for instance on thepiezoelectric element or a shunting circuit) or conversely, by thefraction of input electrical energy that is converted into strain energyin the actuation induced deformation of the face.

Concept 1

In this face coupling embodiment an actuator, 21, capable of planar sizechanges, (also called a 3-1 actuator, although a variety ofinterdigitated piezoelectric wafer or composite actuators are capable ofplanar size changes) is coupled to the plane of the face, 10, onto orburied within the face itself. The actuator can also be packaged usingtechniques known in the art. Since the actuator is not exactly on thecenterline, it couples into bending deformation of the face and acts toimpact a bending moment on the face, 105, when electrically excited.Alternately for in plane actuators near the centerline coupledpreferably into in plane deformation rather than bending, coupling intoout-of-plane motion can be achieved in large deformation scenarios usingparametric forcing. The actuation loading can be thought of as acombination of in-plane forces and a curvature moment couple, 105,acting on the face at the boundaries of the actuator as is shown inFIG. 1. Some critical parameters are the spatial extent (length) of theactuation element as well as its thickness. The spatial x-y extent isdetermined by maximizing the coupling into a given desired facedeformation shape. Good coupling can be equated to the integration ofthe transverse strain field times the electric field times thepiezoelectric constants over the domain of the actuator. The couplinginto some shapes and therefore some structural modes is maximized atcorresponding actuator shapes and extents.

For example, for an axially symmetric plate with a circular actuationpatch covering a given radius, coupling into the second axisymmetricplate mode (one nodal circle) is maximized when the extent of theactuation disk extends to that node radius but no further. If the diskhad a radius larger than the nodal circle's, then material outside thecircle would see strain of opposite sign to the material inside thecircle and there would be a partial cancellation of the piezoelectricresponse when integrated over the entire disk.

For the particular case in which a transducer is coupled and it isdesired to harvest energy from impact as well as potentially excite ahigh frequency mode (to control friction), the actuator must be designedin extent and thickness to achieve both: 1) coupling into the shapeproduced by the impacting ball (roughly the first mode deformation shapefor center hits); and 2) coupling into the deformation shape associatedwith a high frequency mode.

Because faces are relatively thick structural elements, modelingsuggests relatively thick piezoelectric elements on the order of 1 mmare required to produce significant actuation of the 2-3 mm face.Typical face designs have shown that a piezo element a few centimetersin diameter (1-5) can achieve the desired dual objective of coupling toboth the energy generating first impact shape as well as a highfrequency mode to be excited for friction control. A typicalimplementation of this type of face coupling is a 3-1 mode piezoelectricdisk with electric field applied through its thickness and disk directlybonded to the face 10 (usually inside).

It is important to note that the piezoelectric element 21 can beprepackaged with polymer encapsulation and potential electrode patternson such polymer or flex circuit. The patterns can define various activeregions and produce segmented, uniform, or interdigitated electrodepatterns in potentially curvilinear arrays. The key factor is tomaximize electromechanical coupling (as defined above) between thepiezoelectric and the face deformation.

Concept 2

The preferred method and system for coupling of an actuator ortransducer to a face will now be described. In this method the actuationelement 21 (preferably piezoelectric, but possibly electrostrictive ormagnetostrictive or any of a number of actuation or transducertechnologies described previously) is attached to the face though theuse of a housing 12 or support structure attached to the face. Aparticular depiction is shown in FIG. 2 a and in sectioned assembly inFIG. 2 b.

In this case the actuation element 21 is configured to elongate orchange size axially in response to input electrical energy (voltage orcurrent). For a piezoelectric system this can be accomplished in avariety of ways. In particular, one can use a piezoelectric stack tocouple applied voltage to length changes. This is known as 3-3 couplingand is a high mode of response of piezoelectric materials. A 3-3 stackis an arrangement of multiple piezo material layers with electrodesbetween the layers so that the electric field is aligned with a centralaxis to produce a longitudinal piezoelectric effect. This is shown indetail as subassembly 15 in FIG. 18. The actuator can also be configuredas elongated transverse or 3-1 type actuator in which the field isapplied perpendicularly to the axial direction. This can be achieved bya rod with electrodes along its length on opposite sides, or a tubularactuator with the load being applied along its length and the fieldbeing applied through the wall thickness by electrodes on the inner andouter walls of the tube. There are numerous other axially elongatingactuator/transducer configurations known in the art.

The second element is a housing 12 which serves to mechanically connectthe back end of the actuation element to the face. It serves as a stiffload return path coupling elongation of the actuation to deformation ofthe face. Face deformation causes relative motion between the point(potentially at the face center) where the actuator makes contact andthe point where the housing is attached to the face shown in FIG. 2 a bythe applied forces at these points 106. The stiff housing thentranslates that relative motion into relative motion between the twoends of the actuator. The housing 12 thus acts as a mechanicalattachment which couples the actuator length changes to facedifferential motion (deformation). It is therefore in the elastic classof face couplings.

It is important that the housing be stiff (ideally rigid but at least onthe order of the stiffness of the piezoelectric element), since anyelongation of the housing under actuation loads will reduce the loadtransferred to the face and the resulting face deformation. To see this,one should consider the limiting case of a very flexible housing. Then,as the actuation element starts to elongate, the housing just stretcheswith it with little load and therefore little deformation is inducedinto the face. In reality, the condition generally is that the housingmust be stiffer by at least 1 to 20 times greater than the face under anequal but opposite loading at the housing attachment and the actuatorattachment in order to insure that the load is effectively coupled toface deformation rather than housing elongation. The housing should alsobe as light as possible to avoid adding a large mass and therebysignificantly changing the center of gravity of the head or its inertiatensor.

The housing 12 consists of a conical or cylindrical wall 52 with a backplate 13 that provides a contact with the actuator and a circular endwhich establishes contact with the face at a ring 56. See FIGS. 17-19for detailed drawings of a preferred embodiment of Concept 2. Thehousing 12 can be screw attached 29, brazed or welded to the face, oruse any of a number of other techniques. The end plate can bepermanently attached, machined as one piece with the wall or configuredas a screw part 13 for ease of actuator system assembly and removablefor repair. It is important that all the compliances of the housing,including back face bending and other deformation of the housing, betaken into account when considering its stiffness under actuation loads.That is why a conical structure is very efficient, it reduces thebending of the back plate and provides a more direct load path to theface. Typical dimensions are ˜1 mm for the housing wall 52 and ˜3 mm forthe housing back 13. The transducer assembly 15, consisting ofpiezoelectric layered actuator 21 and end pieces 23, is ˜16 mm long(total) as shown in FIG. 18 (of which 10 mm is active material 21). Thecross-section is a 7 mm×7 mm square stack or a preferred 9 mm diametercircular stack.

Of particular design importance is the selection of the contact pointlocations between the housing and the actuator and the face. If theactuator is arranged to make contact with the center of the face, thehousing can be configured to attach to the face at a selected distanceaway from the center at either discrete points or a continuous(circular) ring at a fixed radius. Selection of this attachment radiusis very important to maximize the performance requirements for a givencontrol application. The end pieces 23 are preferably made of steel oralumina or other very stiff material and have some curvature 26 toprovide a centered point contact with the face 33 and with the back ofthe housing 26 on nearly matching curvature (indentations).

In the particular case of friction control, an objective is to excitehigh frequency oscillations as described above. The diameter must bechosen to satisfy the need for: 1) good coupling to the impactdeformation shape to generate electrical energy; and 2) good coupling toa high frequency mode. This can be accomplished by placing theattachment radius to correspond approximately to the radius of ananti-node of the face mode of interest. The anti-node should havepreferentially opposite deformation direction at the center to maximizerelative motion.

The design considerations in optimization are as follows—if the radiusis too small, the piezo center force and the reaction force are imposedon the face very close together. The face is very stiff between thesespaced points and little motion can be introduced. Conversely, thedifferential deformation between those attachment points under theimpact deformation shape, is very small, since it determined by thecurvature under impact loading, so little voltage is generated atimpact. If the radius is made too large, then there is good coupling tothe impact, but it becomes difficult to build a stiff housing structureand it becomes difficult to generate high amplitudes in a high frequencymode because of housing modes starting to participate, effectivelylowering the dynamic stiffness of the housing. In the preferredembodiment, a diameter of attachment of approximately 35 mm was chosenfor the face ring 56 as optimum for maximizing the dual objective ofcoupling to the ball impact face deformation and coupling into a highfrequency face mode at ˜120 kHz.

In evaluating particular designs it is necessary to take intoconsideration stresses in the face and housing and actuator duringimpact. Very high stress level can lead to low fatigue life of thehousing. In addition, the high compressive stresses imposed on theactuator during ball impact can cause a permanent “depolarization” ofthe material, a permanent reduction in actuator properties. Themechanical system must be analyzed for its loads during a variety ofball impact events to determine that these critical load levels for lifeof the housing or stress induced depolarization of the piezoelectricelement have not been exceeded.

One can either have the piezo at the center or one can use a bolt weldedat the center of the face and use a piezo cylinder or multiplepiezo-elements (for example stacks) radially spaced from the bolt asshown in FIG. 3. One can couple to the lowest impact deformation shapeas well as high frequency mode shape in this configuration. Because ofthe axial arrangements relative to the face normal, it is easy topreload the transducer elements 21 for robustness using a centrallylocated face anchor 205 threaded to accept a preload bolt 206 andbacking plate 212 and its easy to design for desired surface excitationamplitude.

Concept 3

A third embodiment is shown in FIG. 4. In this embodiment the piezo 21acts between the face 10 center and a stiff backing/support structure207. The support structure must be stiff for high reaction force—onorder of 1-10×the stiffness of the face so that actuation inducesdeformation of the face instead of the backing structure. There is apotential to use an intermittent contact between the piezo and the face.Because of the requirements of high stiffness, the backing structuretends to be heavy as well.

In Concept 3 shown in FIG. 4, there is a piezo element 21 configuredbetween the face 10 and backing structure 207 which then passes the faceinterface load to another piece of the club head, i.e. the rear, thebody 11, or the perimeter around the face. When the face moves in abouta millimeter during impact of the ball and therefore compresses thepiezo, it generates a charge and electrical energy that can be used topower the system and for example excite an ultrasonic device. Because itgenerates electrical energy through relative motion and load between theface and backing structure, the design must have a stiff backingstructure to resist the motion of the face and provide high piezoloading. If the backing structure were soft, it would deform with theface under low load and wouldn't actually squeeze or apply load to thepiezo. This would imply poor piezoelectric electromechanical coupling tothe impact.

This concept couples to axial motion (or normal motion) of thedeformation of the face. That can be done by a single stack element orsingle piezoelectric monolithic element with a polling direction and theloading is basically aligned with the surface normal to the face. Inthis configuration the actuator would use the 3-3 mode of actuation. Itcould be a 1-3 mode actuator or it could be a tube with the electrodeson the inner or outer wall of the tube as described for Concept 2. Thestress is therefore in the direction perpendicular to the pollingdirection. The basic reaction force is trying to inhibit motion of theface. The backing structure therefore needs to be stiff to accomplishthis effect. This stiffness requirement can lead to relatively heavystructural elements which can by design be located relatively close tothe CG. The added mass, however, would decrease the moment of inertia ofthe head for a fixed mass head since less mass would be available at theperiphery.

In another embodiment of Concept 3, the piezoelectric element isinitially not in contact with the backing structure. Upon ball impact,the deforming face would bring the piezoelectric into contact with thebacking structure and load the piezoelectric element. The piezoelectricelement for instance attaches to the face which is perhaps a halfmillimeter off from the backing structure. No contact is made until theball hits. In this way the system can be designed so that only highamplitude impacts load the piezoelectric element and trigger the controlfunction. Such impacting has been used to achieve damping in structuralsystems. It can also be used to change effective stiffness and theeffective face reaction in different ball loading scenarios andtherefore for different head speeds. For instance, if there is a smallgap between the face and the backing structure, (even if there is notransducer there) low intensity impacts might leave the faceunsupported, not forcing contact. For high intensity impacts, contactbetween the face and the backing will be established during impact; andthe backing structure will support the face and reduce face deflection

Concept 4—Shear Mode Piezo

In the previous concepts the loading on the piezoelectric element hasbeen primarily in the form of an applied normal stress. In Concept 4,the piezoelectric is loaded in shear and coupled into the electric fieldusing the shear mode of piezoelectric operation. More information onshear mode and the major modes of operation of piezoelectric transducerscan be found in the product literature for Piezo Systems Inc. ofCambridge, Mass. The shear mode piezoelectric element involves shearstresses about the axis of polarization in the material as shown in FIG.5 a For example, if the polarization is in the x direction in thematerial, the shear stresses would be in the x-z plane about the y axisas shown in FIG. 5 a. In this mode of piezoelectric operation, theelectric field, E, is applied perpendicular to the poling axis, x. Thismode of piezoelectric response is sometimes called 1-5 mode ofoperation.

In Concept 4, the mechanism using a shear mode piezo actually works verymuch like a constrained layered damping treatment used commonly fordamping of vibratory response of bending structures. The piezoelectricelement 21 that is intended to be loaded in shear is located between theface and a stiff backing layer called the constraining layer 208. As theface bends under the impact loading as shown in FIG. 5 b, theconstraining layer resists that bending deformation putting theintermediate piezoelectric elements in shear. In Concept 4, one ormultiple shear-mode piezo elements are located between the backingstructure 208 and the face 10 as shown in FIG. 5 b so that as the facebends, it induces a shear stress on the piezo which then can be coupledinto the electrical field by the piezoelectric transducer. In thetypical configuration the electrical field is aligned with the surfacenormal and the 1-5 mode piezoelectric elements are polarized in theplane of the face. For instance one of the elements can be placed oneach side of the plate at points of high curvature, then a bar or plateacting as the constraining layer is bonded between these piezoelectricelements. When the face deforms, the bar tries to keep it from deformingand that puts a large shear load on the piezos using the 1-5 mode ofactuation.

In another embodiment, the shear mode piezoelectric element is a ring,polarized radially outward or inward. The ring can be bonded about thecenter of the face. The electric field would act through the thicknessof the ring between the face and the constraining layer. In thisembodiment, the constraining layer would be a disk with the same outerdiameter as the ring bonded to the ring about its circumference. This isan axisymmetric version of the concepts presented above and acts tocouple drumhead like face motion into the piezoelectric element.

The shear mode of operation is a very effective, very high couplingcoefficient mode of operation for piezo transducers. Couplingcoefficients for 3-3 mode of actuation and 1-5 mode of actuation arevery similar. The coupling coefficient is defined loosely as thefraction of the mechanical energy input that is converted intoelectrical energy under a predefined loading cycle.

Concepts 1, 2, 3, and 4 are elastically coupled systems. The piezo issqueezed because of relative deformation between two parts of an elasticbody. Since the face-piezo system is part of that elastic body,deformation of the face imparts deformation of the piezoelectric. ForConcept 1 as the face (an elastic body) deforms, it deforms the piezobecause it is bonded to the face. Concept 2 uses a support structurehousing which connects to the face at a different place than thepiezoelectric element (e.g., the piezoelectric element contacts the faceat the center and the housing contacts the face in a ring at a definedradius out from the center). Because distinct contact points areestablished, relative motion effectively squeezes the piezo. In thismanner the piezoelectric is coupled into the face motion. In Concept 3,motion of the deformation of the face squeezes the piezo attachedbetween the face and the backing structure. In Concept 4, deformation ofthe face induces a shear stress in the piezoelectric element. All ofthese concepts rely on coupling into the elastic deformation of theface-body structure that represents the head of the golf club. For thisreason these concepts are referred to collectively as having elasticallycoupled transducers.

Concepts 5, 6 and 7—Inertial Coupling Concepts

The next class, consisting of Concepts 5, 6 and 7, represents adifferent way of getting a load into the transducer that utilizesinertial forces during impact. These concepts utilize the load necessaryto accelerate a mass to load a piezoelectric element. The piezo loadingis thus a function of acceleration rather than relative deformation ofthe face. In the simplest embodiment, there is a reaction mass 209(sometimes called a proof mass) and a piezo 21 is attached between thatreaction mass and the face 10 as shown in FIG. 6. The system isanalogous to a mass-spring system with the piezoelectric being theloaded spring. The moving face is analogous to a moving base in thespring-mass system. As the face moves under ball impact, inertial forcesinhibit the motion of the reaction mass and the piezoelectric “spring”is loaded by the differential displacement between the face and themass. As it is loaded, it generates the charge and voltage that can thenbe used to control the face as will be described hereinafter.

In these concepts it is important to tune the mass and piezo “spring” tocouple well with the face motion during impact. In the scenario that theface moves slowly in comparison to the period of the first naturalfrequency of the spring-mass system, there is little relative motionbetween the face and the mass and therefore little piezo loading. Inthis scenario the mass follows the face well since elastic forces of thespring are much larger than the inertial resistance. In the alternatescenario, if the face moves very quickly, the mass can't respond and thepiezoelectric “spring” is squeezed by the amount that the wall moves.Thus the load that the piezo sees and therefore the amount of couplingto face motion depends on the relative mass and spring constant of thesystem and the timescale of the forcing.

To illustrate the system behavior, consider the case when the face ismoved with a ½ sine wave similar to an impact motion, the center of theface moves a distance inward (about 1 mm) under ball loading and comesback to normal position in a certain period of time known as the impactduration. If the impact event takes a ½ millisecond, it would correspondto an input wave form corresponding to one half the cycle of a one kHzinput. If the piezo 21, the mass 209 and the spring (face 10) have anatural frequency which is significantly greater than that one kHz, thatsystem looks like a rigid body under that base (face) motion. In thisscenario, there is not a lot of relative deformation in the piezo. Therelative motion corresponds to the amount of strain the piezo sees andthus the voltage the piezo sees in open circuit. With this as themetric, the achievable open circuit voltage under impact drops off tozero at very low frequency inputs (long duration impacts and stiffpiezo-mass systems). It rises up to a resonant peak when the input iscommensurate with the time constant of the spring mass system with theface held rigid. If the first fundamental mode of the spring mass systemis below the forcing frequency, then as the face moves the piezo getssqueezed by an amount of the relative deformation between the movingface and the inertial mass. This is because the mass in unable to movefast enough to respond to the relatively high frequency face motion.

A typical 1 cm by 1 cm by 1 cm cube piezo with a typical 10 gram mass onthe end, might have a frequency in the 20-40 kHz range. That would betoo stiff to couple well into that ˜1 kHz face motion unless a verylarge reaction mass is used. So what that implies then is that thedesigner must try to create a system where there is smaller mass andmuch smaller effective piezo element stiffness, supporting that mass. Ifwell designed, the mass-piezo natural frequency is commensurate and thuswell coupled into that ball impact.

To achieve this frequency tuning, the designer must soften the piezoelement by either making it thinner or using some mechanism to make iteffectively have a lower spring constant. Concepts 6 and 7 shown inFIGS. 7 and 8 respectively demonstrate some manifestations of this usingmechanically amplified piezoelectric transducer configurations. Theseconcepts act by lowering the effective spring constant of the piezoelement, lower than for a stack element. Stack elements can be verystiff. The mechanical amplification increases piezoelectric transducerstroke while lowering its blocked force, essentially reducing theeffective stiffness of the transducer, lowering the spring stiffnessbetween proof mass or the reaction mass and the wall of the face.

If the surface of the face moves slowly relative to the naturalvibration of the effective piezo spring and mass system, then there isrelatively little deformation of the piezo and little charge buildup. Ifit moves fast relative to the time constant, then the piezo element issqueezed by about the deflection of the face. To get energy into thepiezoelectric transducer, the question is how you design the spring andhow large the mass has to be? If the spring and the mass have a naturalfrequency that's tuned to the time constant of the face motion, forinstance a time constant of a ½ ms, then you want the natural frequencyof that spring mass system to be about 1 kHz, and then loading in thepiezoelectric element is maximized. At high frequency, the mass lookslike more of an inertial reaction mass. The piezoelectric element pushesoff from that reaction mass. This allows excitation of direct surfacemotion in the face by force between the reaction mass 209 and the face10.

Concept 5 has the obvious problem of the piezo tied directly to a masswhich ends up being a very stiff system, requiring a large mass to getthe natural frequency down to the range best suited for ball impactcoupling. There are numerous techniques for lowering the stiffness ofthe piezoelectric by mechanical design. For example, piezo rodsconsisting of very thin small diameter pillars can be embedded in anepoxy to lower the effective stiffness but keep the piezo chargecoefficients in place. That's called a 1-3 piezo composite. A compositealso works well with a particulate composite using a piezoelectricparticulate in epoxy. By selecting the appropriate particulate volumefraction a transducer can be designed to lower the effective materialstiffness. Other ways of lowering the effective piezo spring constantwithout sacrificing coupling coefficient are other configurations of thepiezo system, such as having the piezo element mechanically amplified.Concept 6 shown in FIG. 7 illustrates the general idea of a mechanicalamplifier 210 to lower the effective stiffness of the amplifiedpiezoelectric. There are thousands of different types of mechanicalamplifiers that take very large force and very small stroke piezo motionand turn it into much larger stroke, but lower force output. Basically,the effective coupling coefficient of the mechanically amplified piezois always lower than the effective coupling coefficient of the piezo byitself. Concept 6 represents an approach which uses a concept calledaflex-tensional piezo. In that scenario, axial deformation of the motionamplifier (in the directing perpendicular to the face) createshorizontal motion and deformation of the piezo. As the piezo changessize side to side, (i.e., as the piezo gets longer, shorter), it pushesor pulls between the reaction mass and the face. Amplification ratiosmay be anywhere from a factor of 2 to 100. Very small motion creates avery large motion of the system. A mechanically amplified piezo actuatorproduces higher stroke and lower force output. Therefore a softer springcan be used between the face and the action mass to lower the neededreaction mass, lower than required if you had a piezo without mechanicalamplification.

Concept 7 shown in FIG. 8 is a bender configuration. One possiblemanifestation of the bimorph bender 211 is a rectangular strip with onecentral layer of shim and 2 layers of piezo on either side. Sometimesthere is no shim, just 2 layers of piezo. The piezos are actuated sothat the top expands and the bottom contracts. That causes a bending ofthe element very similar to bending of a bimetallic strip due todifferent coefficients of thermal expansion of the top and bottomlayers. The output of this device 211 is force and deflection of thetips. It's a bending mode actuator that essentially turns a small piezomotion in the plane of the bi-morph into large tip deflectionout-of-plane. It works in a manner similar to the mechanical amplifier.Typically, the bi-morphs have much larger tip deflection than in theaxial stroke piezo. Basically the tip deflection of the beam thatrepresents the bi-morph bender turns into the axial compression ortension on the piezoelectric element. Those are typically 1-3 modeelements where there is a piezo wafer with electrodes and loading in theplane of the bending element. Some have used piezo fiber composite (PFC)actuators for the bimoph piezoelectric layers. These PFCs can beconfigured to put the electric fields in the plane of the system usinginter-digitated electrodes and the fibers in the plane of the system tocouple to the planar fields. Two piezo fiber composites can be attached(bonded or laminated) onto each other and can be configured as abi-morph bender. It's an element with high coupling coefficient but hasmuch better force deflection characteristics. In this concept, thebi-morph is typically situated between the proof mass 209 and the facestructure 10.

FIG. 8 shows the single bi-morph in a proof mass off to the side. Youcould have two on opposite sides. Bi-morph transducers have propertiesmaking them efficient as electromechanical transducers. Instead ofhaving a beam pure rectangular plane form so the beam is constant width,the width and/or thickness of the bimorph can be changed as a functionof the length along the beam. It actually is advantageous to taper thebimorph so that it's wider at the base and reduces down to a muchnarrower platform at the point where the load is applied. This works asa more efficiently coupled system to tip motion. Also it's advantageousto change the thickness of the beam as a function of it's position alongthe length of the bi-morph. It is best to have the thicker beam at theroot and thinner beam at the outside. That maximizes the stress in thedevice and minimizes the mass of the device necessary to achieve astated level of energy coupling. You equalize the stress level of thepiezo so you don't have one highly loaded section of the piezo and onevery lightly loaded section. Relatively uniform loading increases itseffective coupling coefficient.

The bi-morphs don't have to be rectangular elements. They could betapered or round. They could have variable thickness. They have alsobeen fabricated as curved structures. There are many differentconfigurations for piezo bi-morphs. Of particular note is thepossibility of a disk shaped (round) bimoph configuration. Thepiezoelectric bimorph disc is attached at the center of the disk to theface with a standoff. The proof mass is a ring attached at the outerradius of the piezoelectric bimorph. The electrodes on the bimorph canbe axis-symmetric and uniform or sectored circumferentially (pie pieceshaped sectors) so that differential tilt can be actuated/responded toby the piezoelectric element.

The Concept 5 embodiment is shown in FIG. 6. The piezo 21 acts betweenthe face center 10 and a reaction mass 209 sized such that a firstnatural frequency of the mass on the piezo is commensurate with twicethe impact duration (tuned). This implies a need for amplified or lessstiff piezo if little reaction mass is used. It is a challenge to makethe piezo soft enough to accept high impact energy but stiff enough toimpact high force at high frequency. A heavy reaction mass may berequired.

The Concept 6 embodiment is shown in FIG. 7. This is like Concept 5except one substitutes a mechanically amplified 210 piezoelectricactuator. A motion amplifier 210 converts small piezo motion to largerrelative motion between the face center and the reaction mass. One maysolve an impedance miss-match problem but there is a potentially heavierand more complex mechanism.

The Concept 7 embodiment is shown in FIG. 8. A bimorph bender 211 actsbetween a mass 209 and the center of the face 10. It's like Concepts 5and 6 but uses a bimorph piezo between the face and a mass. It can usean axisymmetric bimorph disk and ring mass. It can use multiplerectangular or triangular shaped bimorphs and masses. One must tune thefirst mass natural frequency to the impact event and then segmentelectrodes to help locate the ball impact on the face. There's anindeterminate high frequency force output.

Concept 8—Actuator Coupled Between Face and Body

The Concept 8 embodiment is shown in FIG. 9. In this embodiment theactuator or transducer 21 with electrical leads 22 is disposed betweenthe body of the club 11 and the face 10. In this manner, loads betweenthe face and the body at impact can be converted into electrical energyby the transducer during impact and the face can be positioned relativeto the body during impact by selective controlled actuation of thetransducer element(s). These actuations can be used to change theposition such as rotation of the face relative to the body to counteractthe rotation induced in the system by eccentric impacts.

There are multiple modes of operations possible with this configurationof the system. The first is quasi-static positioning. In this mode ofoperation, the face is repositioned from its initial orientation to analternate position relative to the body and ball. For instance, the faceangle is adjusted slightly in off-center impact events. The angleadjustment is pre-calibrated to achieve a reduction in miss distance—forinstance compensating for a hook or slice by re-pointing the face. Thebenefit is accrued by changing the static (with respect to the impactevent) positioning of the face.

In an alternate mode of operation, the face is repositioned during theimpact event so that the induced motion itself causes a desirable effecton the impact outcome. For instance, the face can be moved tangentially(perpendicular to the face normal) such that the face tangentialvelocity during impact beneficially effects the ball spin through thefrictional interface between the ball and the now tangentially movingsurface. The face can be forced to have a tangential velocity which hasthe effect of reducing or increasing the ball spin resulting from theimpact event. This spin control can have desirable effects on thesubsequent ball flight or ball bounce and roll behavior after it hitsthe ground.

In a particular example, the face can be moved upward tangentially tothe face normal axis during the impact event. This can be controlled tooccur only in high impact events that would otherwise produce too high aspin during impact. That too high spin can result in excess lift anddecreased flight distance as is known in the art. The velocity of theupward motion can be a fraction of the ball tangential velocity in thissame coordinate frame. In this case there will be less relative motionbetween the ball surface and the face surface resulting in less spin upof the ball during impact and therefore more distance during flight.

The Currently Preferred Embodiment (Concept 2) Principle of Operation

As the ultimate design goal, the head is designed to convert impactenergy into high frequency, high amplitude vibrations of the club face.High frequency excitation of the face reduces face/ball effectivefriction coefficient using the techniques disclosed in the Katoh andAdachi references and known in the art. The reduction in the effectiveball/face friction coefficient during the face oscillation, acts toreduce ball spin induced by frictional contact with the face at impact.Simulations of ball flight have shown that reduced ball spin resultingfrom impact leads to increased ball travel in a high effective ballvelocity scenario. These scenarios are those associated with higheffective ball velocities i.e.—high head speed and/or high headwind. Inthese conditions the excess lift caused by high spin on the ball resultsin a ballooning trajectory which results in a considerable reduction indown range trajectory. Studies have shown that a 25% reduction in ballspin can increase down range flight distance by 10-20 yards in some highrelative velocity scenarios.

Reduced friction between the ball and the face can also result inreduced sidespin on the ball resulting from impact. Reduced ballsidespin leads to reduced cross range scatter and increased accuracy inthe drive. It is therefore the intention of the invention to provide asystem that can impart the requisite surface oscillations on theclubface so as to achieve the known desirable benefits of controlledspin reduction. The system is controlled in the sense that only the highvelocity impacts (those which exhibit the undesirable excess spin) willtrigger the spin reducing oscillations. It is furthermore the intentionof the invention to power this controlled friction reduction systementirely from the energy available at impact between the golf club headand the ball thereby requiring no external power supply such as abattery.

Simulations indicate the ability of a high frequency driven club faceoscillating with a 5-10 micron amplitude near or above 120 kHz todramatically lower ball spin rate. Simulations of a ball—club impact areshown in FIGS. 12 and 13. FIG. 12 shows the voltage time history of apiezoelectric transducer coupled to the face during impact. The voltagerises until it reaches a critical trigger level (set in the electronics)at which point an oscillation is excited which is tuned to the face modeof interest (120 Kz). These high frequency oscillations are shown inFIG. 13 to reduce the friction coefficient and tangential force betweenthe ball and the face—thereby reducing the rate of spinup at impact andthe resulting ball spin. Curve C in FIG. 13 shows the voltage timehistory analogous to that shown in FIG. 12. FIG. 13B shows thetangential (friction) force between the ball and the face indicating thereduction afforded by the high frequency oscillation in C. The ball spinrate is shown in 13E wherein the ball spin does not increase during thetime that the tangential force is reduced due to the oscillations of theface. The effect is predicated on the hitting surface reaching acritical peak acceleration during the oscillation cycle. The criticalparameter for friction reduction is that the hitting surface (clubface)has to intermittently break contact with the impacting ball. For that tohappen in a ball-face impact scenario, the acceleration of the face awayfrom the ball has to be large enough to break that contact. In effect,the face must move out from under the ball. This only needs to happenfor a short fraction of the impact event in order to effect theball-face friction as shown in FIG. 13. Since during the ball-faceimpact there is a high preload, there is a high compressive load betweenthe ball and the head, shown in FIG. 13A. This ball-face normal loadcauses the ball to accelerate in the direction of the eventual ballflight. The ball is initially at rest and then it has to undergo a highacceleration rate to reach its peak velocity after the impact event. Inorder to break contact, the face must accelerate at a level on the orderof this ball acceleration for at least a portion of the cycle.

The face has to reach a sufficient acceleration backwards away from theball in order to break contact. The amplitude of oscillatory motion ofthe face times the frequency of that oscillatory motion squared isproportional to the peak surface acceleration. It has been found thatsurface oscillatory motions in the range of 5-20 microns amplitude atfrequencies in the 50-120+KHz range have sufficient surface accelerationto break the contact between the face and the ball in a wide range ofimpact conditions. Lower surface motion amplitudes are needed if theoscillation occurs at higher frequency (all else being equal)

When this occurs, the face moves back away from the ball at very highacceleration rates for very brief periods of time. The principle ofoperation is that the induced surface motion has a great enoughamplitude and frequency and the surface acceleration will be high enoughto overcome the compressive loading due to ball impact and actuallybreak contact between the ball and face. The face actually moves awayfrom the ball surface faster than the ball can respond to the loweringof interface force. It moves out from underneath the ball.

The breaking of contact resets the micro-slip region used in a commonmodel of interfacial friction. In this friction model (Katoh) shown inFIG. 20, there is a small amount of relative tangential motion, u,allowed between the bodies (surfaces) before the friction forces buildup to the levels associated with Coulomb (sliding) friction. FIG. 20which is a plot of effective friction coefficient (tangentialcoefficient), φ_(t), as a function of relative displacement between thebodies u. This region of lowering frictional coefficient is due totangential elasticity at the interface. As the surfaces slide past eachother, the friction grows rapidly (in the course of a few micronstravel, noted by u₁ in FIG. 20) up to the asymptotic level associatedwith the Coulomb friction between two sliding surfaces. This frictionmodel represents micro-deformation that occurs to accommodate therelative motion between the surfaces before the interfaces begin toslip. This interface model is presented in the Adachi-reference.

By breaking contact between the ball and face repetitively before theobjects have had enough relative motion to be in the asymptotic region,the sliding between the surfaces occurs only in the micro-slip regionwhich has much lower effective coefficient of friction. Over multiplecycles of breaking contact, the sliding motion is therefore integratedto a lower average friction coefficient between the ball and the face.

There are number of dynamic interactions which occur during theball—face impact. The forces can be thought of as active normal to theface and tangential to the face. Normal forces act through the center ofmass of the ball and so to first order accelerate the ball and do notdirectly induce spin. The tangential forces that arise from the frictionbetween the ball and the face act both to affect the tangentialcomponent of velocity as well as the ball spin.

In the tangential direction during the course of the impact event, theball is starting a slide up the face as it starts to roll. By the timeit leaves the face it's usually rolling up the face with little slidingcomponent, i.e. the ball is rolling (spinning) at a rate such that thepoint of contact at the surface of the ball and the face is not movingrelative to the face contact point. By controlling the effectivecoefficient of friction between the ball and the face, the degree towhich the ball spins up during impact is controlled as shown in FIG. 13trace E If the friction is reduced enough, the tangential forces willnot be sufficient to spin up the ball to the point of pure rolling.Therefore since the tangential (friction) forces directly effect theball spin, controlling these forces can lead to ball spin control.

System Implementation

The system is designed to capture the energy from the ball club headcollision and use it to excite high frequency (ultrasonic) vibrations ofthe face, using these to control friction between the face and ball asdescribed above. It is implemented using piezoelectric elementselastically coupled to face deformations. In the preferred embodimentthe same piezo transducer (in the most general sense as defined forpiezo above) is used both to extract energy from the impact for poweringthe system as well as using the extracted energy to excite ultrasonicvibrations in the club face. In operation, the impact deforms the clubface onto which the piezoelectric transducer is elastically coupled suchthat face deformations are converted to electrical energy (charge andvoltage on the piezoelectric element) for example the elements P10 orP11 in FIG. 10. The electronics that are coupled to the piezoelectrictransducer are configured such that the piezo is initially in the opencircuit condition while it is charging up during the impact. At somepoint the piezoelectric voltage reaches a critical level (trigger level)pre-defined in the system at which point a switch Q10 or Q11 in FIG. 10is closed thereby connecting an inductor L10 or L11 across thepiezoelectric electrodes. The inductor is configured such that theresulting LRC circuit (the C being the capacitance of the piezoelectricelement, and the L being the shunt inductor) responds in an oscillation(ring down) that initiates upon connection of the inductor circuitacross the piezo electrodes. The component values are selected such thatthe frequency of the ring down is approximately tuned (as describedbelow) to a high frequency dynamic structural mode of the face/piezosystem such as the mode highlighted in the frequency response functionin FIG. 22—thereby causing high frequency face motion/oscillation byvirtue of the piezo electro-mechanical coupling. The system is designedsuch that the high frequency face motion is sufficient to control thefriction between the ball and the face as described above.

The system has a number of design issues that will now be discussed. Thesystem is designed to maximally charge up the piezo to obtain maximumelectrical energy stored in the piezo capacitance prior to initiation ofthe ring down/oscillation. This maximizes the oscillation amplitude. Inaddition the system is designed structurally and electrically so as tomaximize the coupling of the piezoelectric to high frequency face motionas will be described below.

Piezoelectric element (21) shown in FIGS. 2 a and 2 b is elasticallycoupled to high frequency face mode so as to excite high frequencyvibrations. The electrical circuit is designed to harvest the impactelectrical energy and use it to drive an oscillator approximately tunedto the selected the face modal frequency. The electronics convert asmall portion of the impact energy into high frequency oscillations ofthe clubface. As the piezo charges up, when it reaches a threshold(trigger level), the control switch (Q10 and Q11 in FIG. 10 and Q3 inFIG. 11 is turned on shunting an inductor across the previously opencircuit piezoelectric and initiating a high frequency oscillation at thefrequency determined by the inductor and piezoelectric capacitance asillustrated in FIG. 12.

The frequency is determined by an LC time constant. The inductor issized for high frequency resonance and should have very low resistanceto reduce energy loss, and appropriate magnetic core or air core toreduce magnetic hysteresis loss and magnetic field saturation effects.The switch can most easily be implemented with MOSFET transistorsalthough other switches with the characteristics of potentially rapidturn on time (sub 1 microsecond) and low resistance when closed. Thereare many other desirable characteristics of the switch which will bediscussed hereinafter.

Face and Piezoelectric Design

The piezoelectric transducer is coupled to the face motion such thatdeformation of the face results in piezoelectric voltages and charges.The objective of the design is to maximally couple the piezoelectrictransducer simultaneously to achieve two effects: 1) maximum coupling(and resulting voltages) to face deformations resulting from ball impacton the face—both impacts at the center of the face as well as impactsoff center, and 2) maximum coupling to a high frequency mode ofoscillation of the coupled piezo-face structural system. The couplingfrom face loading to the piezoelectric open circuit (OC) voltage isrepresented in FIG. 21 which shows the transfer function from adistributed loading representing a ball impact to the piezoelectric opencircuit voltage. The curve represents the response to center hits andthere is a different curve for each hit location located 0.5 in from thecenter location in each of the squared directions (above=north,below=south, toe-ward=west, heel-ward=east). The quasi-static opencircuit voltage for a 10,000 N loading proportional to a 95 MPH headswing is represented by the lower frequency asymptote of the transferfunction noted in FIG. 21. This figure of merit (FOM) can be averagedover a series of hit locations to yield a design FOM that attempts tomaximize the piezoelectric voltage that is generated by a range ofcenter and off center hits.

The coupling to high frequency face mechanical oscillations isrepresented by the transfer function in FIG. 22. This figure representsthe transfer function from applied sinusoidal piezoelectric voltage toface surface acceleration at the center of the face (and at points 0.5inches away in each of the before noted directions). In a like manner tothe voltage response transfer function mentioned above in FIG. 22, themotion/acceleration at a range of locations can be used as the figure ofmerit for the design—averaged or weighted. As is seen, the highfrequency acceleration response is maximized at a vibration mode of theface and coupled piezoelectric system (“Excited mode” in FIG. 22). Inthe preferred embodiment this mode occurs at 127 KHz. Driving the faceat this frequency will maximize surface acceleration. In a like manner,a ring down of the piezoelectric oscillating in the range of frequenciesassociated with the high acceleration response will lead to maximalsurface acceleration.

The goal in the design is to maximize both achieved open circuit voltagedue to center and off-center hits as well as to maximize surfaceacceleration during the subsequent ring down response from this voltageafter the circuit has been triggered. The geometry of the system isselected to maximize these two figures of merit resulting in maximalhigh frequency response of the surface due to the system activation.

The piezoelectric element, club face, and conical housing elementsdescribed below are all configured such that the resulting coupledsystem exhibits these qualities. It is a coupled system design since thesurface response to impacts and resulting voltages are a function of thehousing, piezoelectric transducer, as well as the face geometry andmaterial. In addition, the high frequency mode shapes and frequenciesare very much a function of all three elements of the design. In thefollowing sections, the piezoelectric transducer will be describedfollowed by the housing and face structures.

Stack and Endcap Design

The piezoelectric element is shown in exploded view of the facesubassembly in FIG. 18 and in section view of the face subassembly inFIG. 19. The piezoelectric stack itself is denoted as element 21 whilethe actuator assembly consisting of the stack 21 leads 22, stack endcaps 23 and strain relief 25 is together taken as subassembly 15 in FIG.18. The piezoelectric actuator 21 is preferably configured as amulti-layer stack, 3-3 type actuator. It can alternately be a monolithicrod, tube, or bar, such that electrical input generates axial actuation(motion and stress) predominately and conversely axial loads generatevoltage and charge on the element. Note that 1-3 (transverse) coupledtube or system also has this effect but using a 3-3 stack minimizesvoltages because the layers can be made thin and the 3-3 modemulti-layer stack utilizes the high piezoelectric coupling coefficientsassociated with the 3-3 mode of operation. A centrally positionedpiezoelectric stack is placed between the face 10 and a backing plate(cap 13) that is structurally coupled to the face at carefullydetermined locations. The piezo stack has convex endcaps 23 that providea point contact with the face thereby minimizing bending moments inducedon the stack due to eccentric placement in the system. This is importantin this highly stressed system since it is desirable to operate thepiezoelectric near its maximum allowable stress to minimize systemweight while maximizing electromechanical coupling. In addition, theconvex endcaps 26 are designed so as to distribute the stress moreuniformly though the stack resulting in more ideal stack operation andminimizing stress inhomogeneity in the stack which can cause fracture orinduce stack failure under impact. The endcap thickness is determined toensure sufficient homogeneity. In the preferred embodiment, the endcapshave a radius of curvature of 12.5 mm on the rounded end and measure 3mm from the top to the interface with the piezoelectric stack, They areformed of a stiff material such as alumina or steel to more efficientlydistribute the stress to the stack in a minimal thickness/mass part.Alternately they can be composed of laminations of these materials forease of fabrication.

The stacks 21 consist of co-fired multilayer piezoelectric elements withlayer thickness in the range from 15 to 150+ microns. The systems withthinner layers have much higher capacitance and thereby have a lowernecessary inductance for tuning to a given frequency than the systemusing thicker layers. For example, for a 9 mm diameter circular stack of1 cm total length, if it is assembled from 90 micron layers then thestack capacitance=550 nF, while if it is assembled from 35 micron layersthen the stack capacitance=3442 nF.

The stacks with thinner layers conversely also have much higher currentduring triggering. The higher current can lead to excess loss. Thethinner layers also lead to lower voltage systems under comparablestresses that can simplify and lighten the electronics design. Thepreferred embodiment uses 90-100 micron thick layers. The piezoelectricmaterial is a “hard” composition similar to typical PZT-4. It isselected so as to minimize piezoelectric hysteretic losses as well asmaximize stack robustness and tolerance to high axial stresses duringimpact. The leads are attached such that all the piezoelectric layersact in parallel. The leads are attached to the side of the stack asshown in FIG. 18. The piezoelectric element is ˜1 cm long and 9 mm indiameter. It is attached with a strong epoxy to the curved endcaps witha very thin layer (so as to maximize coupling) such that the overallpiezo/endcap assembly 15 is ˜16 mm long.

Face and Cone Design

The objective is to couple to the face deformation during impact tomaximize generated voltage and charge during impact (generatedelectrical energy) and also couple to a high frequency mode of the facesystem which can be excited by high frequency oscillations of theactuator. The system converts impact energy into high frequencyoscillation of the face. High frequency face oscillation can be used tocontrol the frictional interface between the ball and the face usingconcepts of reduction in interface friction by surface vibration.

The face structure is titanium of carefully controlled thickness so asto create the desirable modal structure having a high frequency modeeasily excited by the piezoelectric element. The general configurationof the face, housing and piezoelectric (together the face assembly 14)is shown in assembled view in FIG. 17, exploded view in FIG. 18 andsection view in FIG. 19. It consists of a piezoelectric element 21 withendcaps 23 (described above) attached to the face 10 and loaded againstit, by a conical housing structure 12. The piezoelectric elementinterfaces to the face at the center point for impacts 33. The face ismanufactured with a small dimple 33 with a radius of curvature slightlylarger than that of the endcap, around 13 mm, so as to provide forpositive location of the stack on the face.

A conical housing 12 with an optional threaded independent endpiece 13is configured to interface with the distal end of the piezo/endcapactuator assembly 15 (opposite the face end). It likewise has a curvedinterface to provide for positive location of the piezoelectric endcap.The conical endcap has a threaded base 29 that screws into the threadedring 37 on the face of the club 10 (inside surface) as shown. Bythreading the cone onto the face, the piezoelectric element ismechanically coupled to the face, and piezoelectric axial size changesare coupled to the face bending. The radius of the ring 56 as well asthe thickness and geometry of the conical housing are carefullydetermined so as to minimize elastic losses and deformation between theface and the distal end of the piezoelectric element. The axialstiffness of the housing must be as high as possible to maximizepiezoelectric coupling to the face deformation.

The conical housing can be configured with access holes in its sides asshown in FIG. 18 element 32. These allow stack positioning and leadegress to the electronics located elsewhere inside the club head. Caremust be taken in the structural design on the face, conical housing, andpiezoelectric element so as to avoid critical stress levels in thesecomponents under the repeated high impact loads. The system is designedso that the housing can be screwed onto the face to press thepiezoelectric stack securely onto the face and provide a sufficientlyhigh compressive preload on the piezoelectric element. The goal is tokeep the actuation element in compression during impact and operationsince piezoelectric elements do not have high tensile strengths.

The face thickness is 2.4 mm inside the cone ring 39 and 2.7 mm outsidethe ring in a step 35 with a gradual taper 36 down to 2.2 mm minimalthickness 34 moving radial outward from the ring. Higher thicknessoutside the ring is due to the increased stress due to the stiff conicalhousing, necessitating thicker walls in these areas. The threaded ringcan be welded onto or formed with the face. It is approximately 2 mmthick and 3.5 mm high, at 38. The conical housing 12 wall thickness isapproximately 1 mm.

A critical dimension is the diameter of the housing at the faceattachment ring 38. This diameter is chosen as large as possible whilestill allowing the system to have a clean axisymmetric vibration mode ata high enough frequency so as to allow excitation of high accelerationsin the face structure. In the preferred embodiment the ring 38 hasapproximately a 35 mm diameter and a height of 4 mm. The face thicknessinside the ring, 39, is 2.4 mm and is chosen to match one of itscomponent modes (as if it were a circular plate vibrating unattached tothe piezoelectric) to the first axial extension mode of thepiezoelectric element. This face—piezo mode matching creates a coupledsystem (once the piezo is attached to the face) which has a high modalamplitude at that design frequency.

The conical housing may have a threaded endcap 13 at its distal end, thehousing threaded surface 30 mating with the endcap threaded surface 27.The opening in the housing allows for a simplified assembly process.With the removable endcap design, the conical housing is attached to theface first. Then the piezoelectric element is inserted and the endcapscrewed onto the conical housing preloading the piezoelectric againstthe face. The endcap can have a concave curved surface to mate with thepiezoelectric convex endcap. The endcap 13 can have a threadedattachment 27 to the conical housing 12.

Electrical Circuitry

The general system is one which converts electrical energy—which hasbeen “quasi-statically” generated during impact by an elasticallycoupled piezoelectric element which is loaded during impact. As thestress/load is applied to a piezoelectric element, the voltage andstored electrical energy builds up on that piezoelectric element. Theelectronics shown in FIG. 10 and FIG. 11 convert that stored electricalenergy on the piezoelectric element, into a high frequency oscillatorymotion of the piezoelectric element. To accomplish this conversion,there is a “switching-event” that switches an inductor L1 in FIG. 11 andL10 or L11 in FIG. 10 across the electrodes of the charged piezoelectricelement at a predetermined voltage threshold. The voltage level can bepredetermined to correspond to an impact of a certain magnitude orintensity and thereby only trigger the system in the event of asufficiently intense impact so as to warrant corrective action on thespin of the ball.

The switch can also be triggered by events other than a critical voltagelevel. For instance the trigger can occur at the peak of the loadingduring impact by using peak detection circuitry which initiates when thepiezoelectric voltage starts to retreat from its previous value (peakdetection circuitry).

The inductor is sized such that the capacitor and the inductor oscillateat a predetermined frequency, (on the order of 120 KHz). Piezoelectricelement capacitance is approximately 480 nF-600 nF, for 100 micron layerthickness at 9 mm diameter and 1 cm total length of the stack. In thissystem the optimal inductor L10, L11, L1 value is ˜1-10 microHenries.

In summary, the circuit design, from a high level functionality, is suchthat it will sense voltage level on the piezo when the piezo electrodesare open circuit, and then at a predetermined voltage level will close aswitch connecting an inductor to that circuit thereby causing the piezo(which has voltage on it prior to triggering) to oscillate at highfrequencies as the voltage and charge on the piezo discharge through theinductor which causes a ringing as shown in FIG. 12.

The circuits depicted in FIGS. 10, and 11 have this simple functionalityof a triggered switch. As the transducer (piezoelectric) is stressedduring the impact, charge and voltage build up on its electrodes,essentially storing the mechanical energy of impact that has beenconverted by the transducer into electrical energy. The particularcircuit operates so that when the voltage reaches a critical threshold,a switch is closed to connect the capacitive piezoelectric element to aninductor. The inductor is sized such that the LC time constant of theclosed electrical circuit (the electrical resonance frequency) is verynear the resonant frequency of a structural mode—in this case theselected face flexural mode.

The high frequency ringing must be as efficient as possible inconverting “quasi-static” energy in the piezo capacitor into the energyof the oscillation. This requires a very low loss oscillation, so thatthe ring-down has very low damping ratio, very high quality factortypically less then 10% of critical, preferably less than 5% ofcritical. This, in turn, requires very low “on” resistance switches andvery low—no loss elements such as low loss inductors and no resistors inthe primary connection path.

High performance in the system also implies avoidance of any parasiticloses. A typical parasitic loss is due to the charge necessary to drivethe switch control circuitry or any electrical system elements such ascapacitors that act to reduce the open circuit voltage that the piezowould normally be generating at impact.

Typical voltage expected to be seen on the piezo before triggering is onthe order of 400 v (system could see 100 v to 600 v). A lot of thesecomponents are going to be high voltage components, and therefore musthave high breakdown voltages but at the same time very low onresistances for very little losses.

So in general the system consists of four components: 1) a piezoelectrictransducer 21 with some capacitance, 2) a switch Q3 in FIG. 11 that iscontrolled by 3) control circuitry, and which connects an 3) inductor L1in FIG. 11 across the piezoelectric electrodes.

It is very important that this main switch turns on very fast when thevoltage on the piezoelectric element electrodes reaches a critical level(pre-determined threshold level). Having the switch turn on fast isimportant for reducing losses because at 120 kHz if it turns onrelatively slowly, if it were to take a few micro-seconds to turn on,the loss in piezo voltage before a true ringdown could occur can bequite substantial. In essence the piezo charge is bled off prior tofully connecting the inductor. This severely limits the initial andsubsequent voltages of the oscillation. An ideal circuit connects theinductor onto the piezo with little or no drop in piezo voltage from itsoriginal open circuit state (prior to the initiation of the switching).In summary, in operation the system reaches a trigger threshold leveland then rapidly closes a high voltage switch so that it has very littleloss and the ringdown initiates at the open circuit voltage leveldetermined by the trigger event.

The block diagram of the circuit is shown in FIG. 10 a and b showing thecontrol circuit driving the switch to connect the inductor element tothe terminals of the piezoelectric element. FIG. 10 a shows aconfiguration in which the switch is between the piezoelectric and theinductor (high side) while 10 b is a configuration in which the switchdrain is nominally at ground (low side). The detailed circuit of theconfiguration in 10 b is shown in FIG. 11. In the following section, itsoperation will be described making reference to the element numbersfound in that figure. The operation of the principal components of thecircuit is as follows:

Piezo (P1):

The circuit is connected to a piezoelectric device P1, with the highelectrode of the piezoelectric device (positive voltage under stackcompression) being connected to inductor L1 (FIG. 11). In FIG. 11, thepiezoelectric element can be represented by a voltage source in serieswith a representative capacitance, C. In actuality these elements arenot part of the circuit and only serve to represent the piezoelectricelement for tuning purposes. This representation neglects the couplingfrom the electrical energy to mechanical energy and really only reflectsthe effects of mechanical forcing on the piezoelectric element(mechanical to electrical coupling). The capacitor C is sized to reflectthe piezoelectric's open circuit capacitance; while the voltage sourceinputs sized to represent the open circuit voltage excursion that thepiezoelectric would see under mechanical forcing in the open circuitcondition (nothing attached). A more complete model for thepiezoelectric would include electrical analogues to the mechanicalproperties such as stiffness and inertia of the piezoelectric device, aswell as a transformer or gyrator coupling the mechanical and electricaldomains.

Inductor (L1):

The inductor, L1, is connected to the piezoelectric element P1. It isinitially floating (not connected to ground) since the switch, Q3, isopen and so no current flows through it. Upon the triggering event andthe subsequent closing of the main switch (Q3), the floating side of L1is connected to ground and a closed circuit is created between thepiezoelectric element and the inductor—now connected in parallel withthe piezoelectric capacitance. This creates a closed LRC circuit, withthe piezo acting as the capacitance, L1 acting as an inductance, and theseries resistance of L1 as well as any on resistance of the main switchQ3 (and any lead resistance) acting as the R. The fundamental goal ofthe design is to create a highly resonant electrical circuit (low R andlow damping) to allow coupling from the electrical oscillations into themechanical oscillations of the piezo and face. For this reason, theinductor must have very low series resistance at the frequency ofoscillation of the LRC circuit. This is typically in the range from50-200 kHz. It is essential to use high quality, low loss inductorsrated for high frequency operation such as in switching power supplies.For our systems, the piezoelectric capacitance is on the order from200-600 nF (with ˜400 nF most typical) and inductance values in therange from 1-12 μH are typically used to set the oscillation frequency(with ˜6 pH most usual) as given by the formula=1/sqrt(LC), where f isthe desired electrical resonance frequency (formula works for lightlydamped systems). In our system we have chosen 3.3 pH power choke coilsfrom Vishay IHLP5050FDRZ3R3M1 or alternately coils from PanasonicPCC-F126F (N6), which for a 8.2 pH value has a DC resistance of ˜11 mΩ(and a very compact package). The tradeoff to be considered is lowresistance vs. package size. Both these weigh about 3 grams each. Sincethe inductance value is typically a function of frequency, it isimportant to select an inductor which has the right value at thefrequency of the resonant circuit.

Since saturation effects can be important upon switching (since thecurrents can be large) care must be taken to choose an inductor whichwill not saturate the core. The saturation changes the effective tuningand inductance value and greatly complicates the tuning process. At highcurrent levels the magnetic fields in the coil saturate, effectivelylowering the coil inductance. This can lead to difficulties in tuningthe resonance, which is now amplitude dependent, and lead to excesslosses on switching since the lower inductance of the saturated inductordoes not act as an effective choke to limit the high currents onswitching. It is desirable to choose an inductor which minimizesnonlinear effects complication tuning, such as saturation and hystereticlosses in the core

Main Switch (Q3):

The main switch is one of the most critical elements of the circuit.When a predetermined threshold voltage is reached, control circuitryturns on the mosfet, Q3, by raising the gate voltage of this N-channelmosfet. Above a critical gate voltage, (˜5-10 volts) the “on” resistanceof the mosfet drops dramatically. The mosfet changes from an opencircuit to a low on-resistance connection to ground for the inductor.Resistor, R4, is sized so that the gate is nominally at ground even inthe presence of a leakage charging current from the mosfet, Q2. When thecontrol circuit fires, the gate of Q3 is rapidly charged up to thethreshold voltage and the “on” resistance of Q3 drops rapidly,essentially closing the switch. Since the charge necessary to fire theswitch is derived from the piezo itself, this firing charge iscompletely parasitic and should be minimized to maximize initial piezovoltage levels. To this effect, a primary requirement of this mosfet isa low gate drive charge and low total gate capacitance. The mosfet alsoneeds to operate at high source-to-drain voltages—i.e., support thepiezo voltage without breakdown prior to reaching the trigger conditionand firing. High breakdown voltage is therefore important. Low onresistance, typically less than 0.1 Ohms is also important since thiscontributes to damping in the electrical oscillation and is perhaps theprimary loss mechanism for electrical energy in the system. It is alsoimportant to note that mosfets have an intrinsic diode from source todrain. This provides a reverse current path during upswings in theelectrical oscillations after switching. In the present circuit, theswitch, Q3, is held on during the electrical oscillations by the diodeD3 which allows charge to flow onto the gate when it fires but not flowoff the gate during subsequent voltage excursions during oscillation.The time constant of how long Q3 stays on after firing is determined bythe combination of the gate capacitance and the resistor R4. Afterfiring, the charge will begin to slowly leak off of the gate until thevoltage threshold is passed, dramatically increasing the drain sourceresistance and in effect opening the switch.

Several high voltage mosfets have been sourced and evaluated there arecurrently two baselines, the APT30M75 from Advanced Power Technologies,and the SI4490 from Vishay Siliconex. Their comparative properties areshown below:

Diode Gate source Ron at Forward Device Vds Max Charge Vg = 10 V voltageAPT30M75 300 V 57nC 0.075 1.3 SI4490 200 V 34nC 0.070 Ohm 0.75These were selected based on their low gate charge and low “on”resistance while still having high voltage capability. For very highvoltage systems, the preferred switch is however the STY60NM50 from STMicroelectronics, rated for 500 volts and 60 amps.

Control Circuitry:

The control circuitry is designed to raise the voltage on the gate of Q3rapidly when a critical threshold voltage level is reached on thepiezoelectric. Rapid turn on (and high gain in the control circuit) isneeded to prevent high energy loss during the transition to the onstate—too slow a transition limits the peak negative voltage excursionof the circuit and the subsequent ringing.

Another feature of the control circuit is that it is latching, meaningthat once Q3 is turned on it stays on regardless of the piezo voltageexcursions. It stays on for a period determined by the leakage of the Q3gate drive charge through R4. R4 is typically 3 megaOhms.

The control circuit operation is as follows: Q3 is initially open so thevoltage at the source terminal (top) of Q3 is essentially the opencircuit voltage of the piezo. At a critical voltage, determined by theZener diodes, D4 D5 and D6, which will collectively start to conduct atthe sum of the rated voltages (plus the diode drop associated with D1)current will start to conduct through D4-D6, charging up capacitor C3and turning on transistor Q1. It is important that D4-D6 be low leakagesince small leakage prematurely through D4-D6 can cause the capacitorC3, to charge up and turn Q1 on partially or prematurely. R2 is sized(typically 100 kOhm) to limit the voltage rise associated with theleakage current of the Zeners, D4-D6, and allow a discharge path forcapacitor C3 (between hits). The transistor Q1, need only be rated forlow voltage since its source is connected to the control supplycapacitor C4 which is maintained at no higher than 28 volts by Zener D2.

The control supply capacitor C4, is charged up during the initial highvoltage excursion of the piezo. It charges with a rate determined byresistor R3 (typically 5 kΩs). In the present system, this is set atabout 5kΩs allowing a charge time of approximately 100-200 μsec for a C4value in the range of about 47 nF. In design, the resistor R3, is sizedfor rapid charge up after the capacitor C4 is sized. The capacitor C4 issized such that when it is connected to the main switch Q3 gate (when Q2switches on) it dumps its charge into the as yet uncharged Q3 gate,lowering the voltage on C4 and raising the gate voltage on Q3 to thefull on condition. Therefore C4 is sized to be large enough to supplythe gate charge of Q3 up to the needed ON level. Since the charge on C4is parasitic to the piezo charge and effectively lowers the piezovoltage, it is desirable to have C4 as small as possible yet stillenable needed gate voltage rise on Q3. For the selected M1 s, this valuecan be as low as 3.3 nF, but for some of the larger main mosfets, 47 nFwas needed. In practice the capacitor C4 peak voltage which is limitedby the Zener, D2, is set as high as practicable while keeping thecontrol mosfets and transistors low cost and low loss. In our circuit wechose 28 volts for the supply capacitor C4. Testing has shown that atthese component values, the control circuits reduced the piezo voltageby only a small fraction of the total open circuit piezo voltage.

When the critical voltage is reached and switch Q1 is turned on, this inturn pulls down the gate of the P channel mosfet Q2, rapidly turning iton and connecting the charged capacitor C4, to the main mosfet Q3 gate.This, in turn, charges the Q3 gate up and turns Q3 on rapidly. AFairchild BSS110 was used for the p-channel mosfet Q2. The mosfetversion of the circuit has much lower leakage through from C4 to the Q3gate. This leakage occurs when C4 is charged but switches Q2 and Q3, arenominally open. This leakage of charge onto the gate of Q3 causedpremature partial switching ON of Q3. Using the mosfet in Q2 eliminatesthis leakage and leads to clean switching. Once the gate of Q3 ischarged up, it stays charged since it charges through diode D3 and onlyswitches back open after the gate charge has drained through R4.

Electrical Concluding Overview: The piezoelectric element, essentiallyand initially an open circuit, charges up. As low parasitic lossesdragging the piezo voltage down reaches some threshold level that isuser controllable, an electrical switch connects an inductor across thepiezo and starts it oscillating at very high frequencies. That switchhas to switch very rapidly to avoid losses during the transition fromopen circuit to closed circuit. It has to have very low on resistanceand a circuit is required that fires that and powers that switch anddoesn't have a lot of capacitive drain because that would lower thevoltage on the piezo. The energy used to turn the switch on, is energynot available for the oscillation.

It is desirable to have the ability to be able to tune, switch out orunder electrical control, switch in and change the inductors to providevariable tuning frequency.

Some circuits have a self-locking oscillation. They automatically fallinto an oscillation frequency determined by feedback gain or delay gainin the circuit. It's possible this would allow locking to the piezovibration.

It has been found useful for the system to have some external interfacesthat allow probing of the voltages and signals in the system duringoperation. Various leads/sensors/probe points (external interfaces fromthe board) allow one to tune and examine the system states andconditions throughout testing and operations. The signals can be carriedout by external wires, etc. without disturbing the system, or can bebrought out wirelessly. The interfaces to external electronics (wired orwireless) can also be used for monitoring/telemetry and also forreprogramming of the system performance or diagnostics and datadownloading.

These electrical circuit elements (external to the piezoelectric elementcoupled to the face) are configured in a single or multiple boards on asingle or multiple sides. The board is preferentially configured insidethe head of the golf club or external to the club, connected bytransducer leads running out of the head to the board as shown in FIGS.13 and 14. Some or all of the components can be located on the externalboard to allow for easy access to the circuitry for changing triggerlevels or other tuning of the circuitry. Alternately, the board 18 canbe configured on a sole plate 54 (or other removable part) as part of asole plate assembly 16 shown in FIGS. 14 and 15 attached to the head andin FIGS. 16 a and 15 b detached from the club head. The sole plateassembly 16 can be configured with leads 22 or plug connectors 20 sothat electrical connection is made on assembly of the removable piece tothe main body of the club. Such an arrangement is shown in FIGS. 14 and15 in section view and in FIGS. 16 a and 16 b with sole plate assemblydetached. These figures illustrate an electrical circuit board 18mounted on a removable sole plate 54 by standoffs 45 such that when thesole plate is inserted and connected to the dub body 11 by fasteners 47,an electrical connection is made between a connector on the primaryboard 49 and a connector 20 on a secondary “connector” board 19 which ispermanently mounted in the head 11 by standoffs 44 and electricallyconnected to the transducer 21 and face assembly 14.

This arrangement allows for the simple removal andtuning/maintenance/repair of the electrical circuit and board. Theconnector and the connector board permanently mounted in the head allowthe simple removal of the primary board. Additional connectors can beconfigured on the primary board to allow for externalmonitoring/diagnostics during club swings and impact. Alternately, suchinformation can be wirelessly transmitted to a receiver and stored forlater examination. Alternately the data taken during the impact eventcan be stored on the board in on-board memory for laterdumping/downloading upon a command prompt. The telemetry transmissioncan occur over wireless or wired channels. Such information that can bestored and monitored includes swing speed, impact force, ball faceimpact location and intensity, club head deceleration and resultant ballacceleration or any of a number of system states that are associatedwith the dynamics and conditions of the club swing and impact (orresulting vibration of response of the ball-head system).

Assembly Procedure

In assembly the sequence of events can proceed in many orders of whichone is presented below.

-   -   1) Form the face 10 with appropriately configured ring. Perform        post forging machining operations to set inner diameter and        thread 37 on the inner diameter of the ring. Also form and        polish the dimple 33 at the location that the stack will        interface when in contact with the face    -   2) Put a dummy threaded piece into face ring thread to hold its        shape and then weld the face onto the body 11. Then remove the        supporting dummy threaded piece.

3) Screw on cone 12 until tight

4) Insert piezo stack/piezo endcap assembly 15 into cone to make contactwith the face. There may be a supporting element made of plastic orother flexible material, designed to hold the piezo in place/positionuntil the endcap of the cone can be screwed on and the piezo can therebybe preloaded and against the face and locked into position. The leads ofthe piezoelectric 22 must be routed through the holes in the housingwalls 32. These should have an appropriate grommet or strain relief toavoid abrasion during impact induced motion.

-   -   5) The endcap 13 is then screwed onto the cone (curved side        interfacing with the piezoelectric stack assembly) until the        piezo is securely seated and preloaded against the face        sufficiently to avail breaking of contact between the face and        the piezo endcaps during impact (around 1000 N compressive        preload). A thin layer of machine oil can be used between the        endcaps of the piezo assembly and the face and the cone endcap        to aid in seating.    -   6) The screw on cone endcap 13 is than locked in place with a        set screw, epoxy or other method of fixation.    -   7) The leads of the piezo are then soldered onto a small        connector board 19 that holds the connector 20 for interfacing        with the primary (removable) board 18. The connector board is        permanently attached into the head with epoxy or screws on a        standoff 44 The connector board is positioned so as to interface        to the primary board without interference.    -   8) The crown of the club head 43 is then bonded to the head body        11 in a 160 degree C. epoxy bonding operation.    -   9) The primary board 18 and connector 49 are attached to the        removable sole plate 54. And the entire removable assembly 17 is        then inserted into the club head and screwed in. The system is        now operational.

Alternate Embodiment Face Stiffness Control

In the forgoing sections, a method and system for achieving face-ballfriction control using ultrasonic vibrations was presented. In thissection an alternate embodiment using a piezo (or other) transducercoupled to a face of a golf club (putter, driver, iron) to effectstiffness control will be presented. By varying the effective facestiffness, the course and result of the ball-face impact iseffected/controlled and so this is generally one example of an impactcontrol system using solid-state transducer materials. The conceptspresented in this section are described in terms of a piezoelectrictransducer coupled to a face but apply more generally to a system withany transducer coupled to face motion—as long as the transducer iscapable of converting mechanical energy to electrical energy and viceversa i.e. exhibits electro-mechanical coupling.

General Principle

The general concept is to utilize the aforementioned electromechanicalcoupling of a face-coupled transducer to change the effective stiffnessof the face under prescribed conditions. In essence, one controls thestiffness of the face to produce a desirable effect from the resultingball—face impact (with the controlled stiffness). The stiffness can becontrolled because in a system with electro-mechanical coupling,changing the boundary conditions on the electrical side (ports) of thesystem changes the effective stiffness of the mechanical side of thesystem. For example, it is well known in the art that the stiffness of ashorted piezoelectric element is lower than the corresponding stiffnessof an equivalent piezoelectric element with the electrodes open. Thiseffect can be used to change the effective stiffness (longitudinal i.e.,in the poling direction or shear mode, i.e., transverse to the polingdirection) of the piezoelectric material and piezoelectric element.Since the piezoelectric element is mechanically coupled to the face,this change in piezoelectric element stiffness results in a change inthe stiffness of the face.

In any of the transducer-face mechanical coupling embodiments presentedabove (Concepts 1-8), the transducer is mechanically coupled to the facein such a way that a change in the stiffness of the transducer changesthe behavior of the face. In the case of the elastically coupledembodiments (Concepts 1-4), it can be said that a change in stiffness ofthe transducer directly changes the stiffness of the face to ballimpact. This equivalently changes the deflection of the face underimpact. In the inertial coupled cases (Concepts 5-8) changes in thetransducer stiffness result in changes to a coupling between the facemotion and an inertial mass (for Concept 8 this is the remainder of theclub head)—changing the dynamic stiffness of the face if not thequasi-static stiffness (DC). This is because these inertial coupledconcepts are not DC coupled. They have no effect on the system at verylow frequencies since there is little inertial force from the proof massat low frequencies. They are designed to have effect on the system atthe impact timescales, however, and so a change in the transducerstiffness in these concepts results in a change in the stiffness of thesystem in the frequency range associated with ball impact (around 0.5milliseconds and 1 kHz). Thus any of the Concepts 1-8 can be used tochange the effective stiffness of the face under impact by varying thestiffness of the transducer.

Transducer Configurations

As mentioned above any of the previously described transducerconfigurations can be used as the basis for this impact control concept.For example, one embodiment uses a piezoelectric stack coupled to theface as in Concept 2. In the mechanical design presented previously forConcept 2 and shown schematically in FIGS. 2 a and 2 b and in detail inFIGS. 13-19, the face DC stiffness (to central ball forces normal to theface) increases approximately 25% from the short circuit case to the andopen circuit scenarios. An alternate configuration to using a stacktransducer is to use a planar (potentially packaged) piezoelectrictransducer (or other solid state transducer material) bonded to the faceand thereby coupled to face motion through coupling to face extensionand bending. The face bending stiffness and thereby overall stiffness tothe ball forces can be changes by changing the electrical circuitboundary conditions (open circuit or short circuit).

System Circuitry Operation

To enable the control, the transducer electrical boundary conditionsmust be determined (controlled) based on some response or behavior ofthe system.

This can be determined based on the transducer itself (i.e., voltage orcharge under loading) or it can be determined by an independent sensorfor example face strain or face deflection sensor. An accelerometer canalso be used to determine club head deceleration under impact andtrigger the system accordingly.

In operation, the transducer is placed into an open circuit or shortcircuit condition depending on the sensor. For example the electricalconnections can be controlled based on impact intensity—making thesystem stiffer under more intense ball impacts and less stiff undersofter ball impact. This can be especially important in conditionsrequiring enhanced feel, longer ball dwell time and an increase intopspin or launch angle such as in putting and putters, or wedges andshort iron shots.

In putting it is known in the art that the key to reducing skid is togive the ball as much topspin as possible before it leaves the putterface and it is advantageous to minimize the distance that the ball skidsbefore it starts to roll.

The impact of a putter compresses the golf ball front to back whilewidening the girth for an instant. The ball then rebounds to its initialshape, causing it to propel forward from the club face. A perfectscenario would have the golf ball rebounding in a direction determinedonly by the direction the putter is traveling and the angle of theputter face relative to that direction. Since golf balls are notperfectly balanced, imperfections in the ball can cause deviation in therebound direction called compression deflection. A reduction to theamount that the ball is compressed at impact reduces compressiondeflection. A softer face reduces interface loading and decreases theball compression. Therefore, when properly tuned the desired effect ofthe system reduces ball compression deflection and optimizes launch androll conditions. For example in putters, the combination of having arelatively soft clubface with a high rebound resilience increasescontrol both in distance and direction.

The elastic deformation of both the ball and face materials has atremendous influence on the direction, velocity and manner a golf ballwill propel, launch or spring from a clubface after being compressedduring the impact event. The effective resilience of a clubface strikinga ball is a combination of the resilience of the ball and clubface. Tomaximize control, in putters and wedges it is better for a substantialportion of the effective resilience to come from the clubface, not fromcompression of the ball, to reduce compression deflection.

In contrast with this desire for more compliance in the face to increasecontrol, in putting and shorter golf shots as the velocity of impactincreases the amount of control could potentially decrease with a morecompliant face due to the intensity of the impact and force of thestroke relative to percussion point. Impact induced deformations cancontribute to ball trajectory errors and stroke inconsistency especiallyin non ideal impacts at high intensity. Essentially the increasedcompliance can lead to a loss of control in higher intensity impactscenarios.

To increase the control of the shot and reduce scatter, it is thereforedesirable to have a clubface which has lower stiffness in lower impactintensity events but higher stiffness in higher impact intensity events.

In the preferred embodiment when the Piezo is in shorted condition andan increase in the amount of time in which the ball remains in contactwith the clubface, “Dwell Time” is coupled to a clubface with highcoefficient of friction, an appreciable increase in control andoptimization of ball launch conditions result.

Increased dwell time enables the clubface an extended opportunity tohold the ball for the purpose of imparting topspin. It is also knownthat a longer dwell time improves feel.

For example in low velocity impacts with a putter the shorted Piezoenables the clubface to cradle the ball during contact, resulting inmore dwell time and less skidding onto the green. Additionally thisperformance characteristic translates to an enhanced feel and controlwhich is also known in the art to improve accuracy, consistency andconfidence.

In contrast stiffening the face in higher velocity impacts can alsoincrease accuracy and consistency by reducing elastic deformationinduced errors. Additionally the variable stiffening effect presents asignificant range of performance characteristics out of one golf clubusing only simple electrical circuit variations. Whereas the same rangeof performance characteristics in a passive golf club design wouldrequire several identically designed golf clubs with varying clubfacematerial boundary conditions to perform at this range. Thus the idea ofa electrically tunable or fittable club system is possible Whereinchanging a resistor or trigger level can be used to change the clubbehavior to match a particular player, or playing condition.

By making the system stiffer under certain conditions during the courseof impact, the impact result is being controlled. Alternately thestiffness change can be configured and fixed by the user prior to theshot, thereby enabling a kind of fitting of the club to the user. Theuser can select the most desirable stiffness setting and have it set atthe factory or in a user controllable system, the stiffness can be setby the user prior to play—depending on the user's desires or conditionof the game (weather, wind, etc). The switch or other electrical settingdevice can be configured for easy user access, for instance at the endof the grip.

A schematic of a preferred embodiment which uses the piezoelectricitself as the impact sensor is shown in FIG. 23.

In operation, the circuit acts to open the piezoelectric electrodes inharder impact scenarios and leave them shorted in softer impactscenarios. The transducer (coupled to the face) is electricallyconnected to charge or voltage sensing circuitry. In essence it isconfigured as a sensor. The sensing circuitry keeps the piezoelectrichigh lead at ground, essentially shorting the piezoelectric. In thiscondition the piezoelectric transducer exhibits short circuit mechanicalproperties. If the sensor output voltage reaches a critical level, thenthe circuit is triggered and the switch (normally closed) which connectsthe piezo to the circuitry is opened, essentially opening the electrodesof the piezoelectric transducer. Upon triggering the electronics, thepiezoelectric transducer then has open-circuit stiffness and the face towhich it is mechanically coupled will now have higher stiffness for theremainder of the impact.

A circuit which implements this is very similar to the circuitrydescribed above for the friction control application. The circuit ismodified by replacing the inductor L1, with a resistor, R12 in FIG. 23,and the switch M1, which is an n-channel enhancement mode mosfet in thefriction control circuit—is replaced with a new mosfet which is ann-channel depletion mode mosfet Q12. With a depletion mode n channelmosfet Q12, the circuit is initially in the short circuit condition i.e.the switch Q12 is closed. Upon lowering the voltage at the mosfet gate(when it triggers) the depletion mode mosfet opens the circuit, therebydisconnecting the resistor and thereby the piezoelectric electrodes. Thecircuit is now open circuit. The control circuit operates to lowerrather than raise the gate voltage as in the friction control circuit.Such voltage driven mosfet drive circuits are common in the art.

The trigger event is set when the voltage on the piezoelectric reaches athreshold voltage sent by the Zener diode. The voltage rises because thepiezo is forced to discharge through the resistor, R12, and thereforenot perfectly shorted. This provides the opportunity to trigger off thevoltage rise that occurs when the piezo is forced. If the piezo weretruly shorted, the voltage would not rise and the trigger would notoccur. Since the piezo is initially shunted by the resistor, R12 (theswitch Q12 being initially closed), the voltage will rise as long as theforcing occurs at a rate on par with or greater than the RC timeconstant of the system. Forcing at frequencies below that associatedwith the RC time constant, the voltage will not rise much since theresistor appears as a short. Above this time constant (i.e., forrelatively rapid forcing) the resistor appears as an open circuit andthe voltage rises. The piezo essentially does not have the time todischarge through the resistor during the course of the event.

The circuit thus has the effect that impacts of sufficient rate orintensity that raise the voltage on the resistor-shunted piezo, triggerthe circuit and open the depletion mode mosfet effectively opening thecircuit and putting the piezo in a open circuit electrical situation.The system thus stiffens the system upon sufficiently intense or rapidimpacts. The system can be tuned by selection or an appropriate shuntingresistor, or (primarily) by selecting the appropriate triggering Zenerbreakdown voltage.

The above mentioned system is self sensing and self powering in that itdraws power from no external source but rather from the charges of theface-coupled transducer itself. It should be noted that the triggeringsignal could be derived from an alternate sensor. In addition thefeedback logic could be more complicated, perhaps even determined by aprogrammable microprocessor. This microprocessor could be powered fromenergy extracted by the circuitry from the impact event. Themicroprocessor could be externally programmed as a result of a fittingsystem to respond under predetermined conditions particular to anindividual golfers characteristics and capabilities. This is the conceptof a programmable smart club designed to maximize the benefit fromimpact derived from a given golfer's swing. The programming essentiallyallows the club behavior to be tuned and customized to the individualgolfer and his characteristics and capabilities. For example correctingfor hooks or slices.

Having thus disclosed various embodiments of the invention, it will nowbe apparent that many additional variations are possible and that thosedescribed therein are only illustrative of the inventive concepts.Accordingly, the scope hereof is not to be limited by the abovedisclosure but only by the claims appended hereto and their equivalents.

1. A golf club head having a hitting surface for impacting a golf ball;the head comprising: a transducer for converting mechanical energy fromsaid surface during a golf ball impact event to electrical energy; acircuit coupled to said transducer for selectively generating atriggering signal responsive to said electrical energy; and an actuatormechanically coupled to said hitting surface and actuated responsive tosaid triggering signal, said actuator mechanically altering said hittingsurface and thereby altering golf ball impact in adaptively controlledmanner during said impact event in response to said electrical energy.2. The golf club head recited in claim 1 wherein at least one of saidtransducer and said actuator comprises a piezoelectric element.
 3. Thegolf club head recited in claim 2 wherein said piezoelectric element isaffixed to said hitting surface.
 4. The golf club head recited in claim3 wherein said piezoelectric element is coupled to said hitting surface.5. The golf club head recited in claim 3 wherein said piezoelectricelement is interconnected to said hitting surface by an intermediatemember.
 6. The golf club head recited in claim 3 wherein saidpiezoelectric element is braced against said hitting surface.
 7. Thegolf club head recited in claim 1 further comprising a support structurewithin said head, said support structure maintaining firm contactbetween said hitting surface and said actuator.
 8. The golf club headrecited in claim 3 further comprising a support structure within saidhead, said support structure maintaining firm contact between saidhitting surface and said piezoelectric element.
 9. The golf club headrecited in claim 7 wherein said support structure comprises a conicallyshaped housing.
 10. The golf club head recited in claim 8 wherein saidsupport structure comprises a conically shaped housing.
 11. The golfclub head recited in claim 1 wherein said actuator is configured tocause said hitting surface to vibrate at a selected frequency while saidgolf ball is being impacted by said hitting surface.
 12. The golf clubhead recited in claim 1 wherein said actuator is configured to causesaid hitting surface to vibrate at an ultra-sonic frequency.
 13. Thegolf club head recited in claim 1 wherein said actuator is configured tocause said hitting surface to vibrate at a frequency and with anamplitude sufficient to interrupt contact between said hitting surfaceand said golf ball.
 14. The golf club head recited in claim 1 whereinsaid circuit comprises a reactive impedance for storing said electricalenergy
 15. The golf club head recited in claim 1 wherein said circuitcomprises a reactance for storing said electrical energy.
 16. The golfclub head recited in claim 1 wherein said circuit comprises an inductorfor storing said electrical energy.
 17. The golf club head recited inclaim 1 wherein said circuit comprises an inductor and a capacitor forstoring said electrical energy.
 18. The golf club head recited in claim1 wherein said circuit comprises a switch for selectively applying saidelectrical energy in response to a threshold parameter of said hittingsurface impacting said golf ball.
 19. The golf club head recited inclaim 18 wherein said parameter is the magnitude of an electricalvoltage produced by said transducer in response to said impacting.
 20. Amethod of reducing the effective coefficient of friction between theface of a golf club head and a golf ball; the method comprising thesteps of: establishing a transducer in said golf club head; and,automatically actuating said transducer to convert the energy upon ballimpact with said face during an impact event into anelectro-mechanically actuated ultra-sonic vibration of said face tothereby mechanically alter said face for the interaction of said faceand said ball in adaptively controlled manner during said impact event.21. The method recited in claim 20 wherein said converting stepcomprises the steps of: converting said ball impact energy intoelectrical energy; and converting said electrical energy into saidultra-sonic vibration.
 22. The method recited in claim 21 wherein saidconverting steps are each carried out using a piezoelectric elementmechanically coupled to said face.
 23. A golf club head comprising: ahitting surface; and, a variable stiffening element coupled to saidhitting surface for automatic electro-mechanical actuation, saidvariable stiffening element selectively increasing and decreasing thestiffness of said hitting surface of said head in adaptively controlledmanner during an impact event responsive to impact with a ball duringsaid impact event.
 24. The golf club head recited in claim 23 whereinsaid stiffening element comprises a piezoelectric element having a firstlevel of stiffness when a short circuit configuration is generatedthereacross and a second level of stiffness when a open circuitconfiguration is generated thereacross.
 25. The golf club head recitedin claim 24 wherein the configuration of said piezoelectric element isdetermined by a switch controlled by a sensor responsive to the level ofimpact of said golf club head with a golf ball.